diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index 0274de6..45998a1 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -2135,10 +2135,10 @@
    see Objects/stringlib/localeutil.h */
 #ifndef Py_LIMITED_API
 PyAPI_FUNC(Py_ssize_t) _PyUnicode_InsertThousandsGrouping(
-    PyObject *unicode,
-    Py_ssize_t index,
+    _PyUnicodeWriter *writer,
     Py_ssize_t n_buffer,
-    void *digits,
+    PyObject *digits,
+    Py_ssize_t d_pos,
     Py_ssize_t n_digits,
     Py_ssize_t min_width,
     const char *grouping,
diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-11-20-22-33-38.bpo-33954.RzSngM.rst b/Misc/NEWS.d/next/Core and Builtins/2018-11-20-22-33-38.bpo-33954.RzSngM.rst
new file mode 100644
index 0000000..9bfbe16
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2018-11-20-22-33-38.bpo-33954.RzSngM.rst
@@ -0,0 +1,3 @@
+For :meth:`str.format`, :meth:`float.__format__` and
+:meth:`complex.__format__` methods for non-ASCII decimal point when using
+the "n" formatter.
diff --git a/Objects/stringlib/localeutil.h b/Objects/stringlib/localeutil.h
index df501ed..31fed34 100644
--- a/Objects/stringlib/localeutil.h
+++ b/Objects/stringlib/localeutil.h
@@ -1,28 +1,24 @@
-/* stringlib: locale related helpers implementation */
-
-#include <locale.h>
-
-#if !STRINGLIB_IS_UNICODE
-#   error "localeutil.h is specific to Unicode"
-#endif
+/* _PyUnicode_InsertThousandsGrouping() helper functions */
 
 typedef struct {
     const char *grouping;
     char previous;
     Py_ssize_t i; /* Where we're currently pointing in grouping. */
-} STRINGLIB(GroupGenerator);
+} GroupGenerator;
+
 
 static void
-STRINGLIB(GroupGenerator_init)(STRINGLIB(GroupGenerator) *self, const char *grouping)
+GroupGenerator_init(GroupGenerator *self, const char *grouping)
 {
     self->grouping = grouping;
     self->i = 0;
     self->previous = 0;
 }
 
+
 /* Returns the next grouping, or 0 to signify end. */
 static Py_ssize_t
-STRINGLIB(GroupGenerator_next)(STRINGLIB(GroupGenerator) *self)
+GroupGenerator_next(GroupGenerator *self)
 {
     /* Note that we don't really do much error checking here. If a
        grouping string contains just CHAR_MAX, for example, then just
@@ -43,138 +39,44 @@
     }
 }
 
+
 /* Fill in some digits, leading zeros, and thousands separator. All
    are optional, depending on when we're called. */
 static void
-STRINGLIB(fill)(STRINGLIB_CHAR **digits_end, STRINGLIB_CHAR **buffer_end,
-     Py_ssize_t n_chars, Py_ssize_t n_zeros, STRINGLIB_CHAR* thousands_sep,
-     Py_ssize_t thousands_sep_len)
+InsertThousandsGrouping_fill(_PyUnicodeWriter *writer, Py_ssize_t *buffer_pos,
+                             PyObject *digits, Py_ssize_t *digits_pos,
+                             Py_ssize_t n_chars, Py_ssize_t n_zeros,
+                             PyObject *thousands_sep, Py_ssize_t thousands_sep_len,
+                             Py_UCS4 *maxchar)
 {
-    Py_ssize_t i;
+    if (!writer) {
+        /* if maxchar > 127, maxchar is already set */
+        if (*maxchar == 127 && thousands_sep) {
+            Py_UCS4 maxchar2 = PyUnicode_MAX_CHAR_VALUE(thousands_sep);
+            *maxchar = Py_MAX(*maxchar, maxchar2);
+        }
+        return;
+    }
 
     if (thousands_sep) {
-        *buffer_end -= thousands_sep_len;
+        *buffer_pos -= thousands_sep_len;
 
         /* Copy the thousands_sep chars into the buffer. */
-        memcpy(*buffer_end, thousands_sep,
-               thousands_sep_len * STRINGLIB_SIZEOF_CHAR);
+        _PyUnicode_FastCopyCharacters(writer->buffer, *buffer_pos,
+                                      thousands_sep, 0,
+                                      thousands_sep_len);
     }
 
-    *buffer_end -= n_chars;
-    *digits_end -= n_chars;
-    memcpy(*buffer_end, *digits_end, n_chars * sizeof(STRINGLIB_CHAR));
+    *buffer_pos -= n_chars;
+    *digits_pos -= n_chars;
+    _PyUnicode_FastCopyCharacters(writer->buffer, *buffer_pos,
+                                  digits, *digits_pos,
+                                  n_chars);
 
-    *buffer_end -= n_zeros;
-    for (i = 0; i < n_zeros; i++)
-        (*buffer_end)[i] = '0';
+    if (n_zeros) {
+        *buffer_pos -= n_zeros;
+        enum PyUnicode_Kind kind = PyUnicode_KIND(writer->buffer);
+        void *data = PyUnicode_DATA(writer->buffer);
+        FILL(kind, data, '0', *buffer_pos, n_zeros);
+    }
 }
-
-/**
- * InsertThousandsGrouping:
- * @buffer: A pointer to the start of a string.
- * @n_buffer: Number of characters in @buffer.
- * @digits: A pointer to the digits we're reading from. If count
- *          is non-NULL, this is unused.
- * @n_digits: The number of digits in the string, in which we want
- *            to put the grouping chars.
- * @min_width: The minimum width of the digits in the output string.
- *             Output will be zero-padded on the left to fill.
- * @grouping: see definition in localeconv().
- * @thousands_sep: see definition in localeconv().
- *
- * There are 2 modes: counting and filling. If @buffer is NULL,
- *  we are in counting mode, else filling mode.
- * If counting, the required buffer size is returned.
- * If filling, we know the buffer will be large enough, so we don't
- *  need to pass in the buffer size.
- * Inserts thousand grouping characters (as defined by grouping and
- *  thousands_sep) into the string between buffer and buffer+n_digits.
- *
- * Return value: 0 on error, else 1.  Note that no error can occur if
- *  count is non-NULL.
- *
- * This name won't be used, the includer of this file should define
- *  it to be the actual function name, based on unicode or string.
- *
- * As closely as possible, this code mimics the logic in decimal.py's
-    _insert_thousands_sep().
- **/
-static Py_ssize_t
-STRINGLIB(InsertThousandsGrouping)(
-    STRINGLIB_CHAR *buffer,
-    Py_ssize_t n_buffer,
-    STRINGLIB_CHAR *digits,
-    Py_ssize_t n_digits,
-    Py_ssize_t min_width,
-    const char *grouping,
-    STRINGLIB_CHAR *thousands_sep,
-    Py_ssize_t thousands_sep_len)
-{
-    Py_ssize_t count = 0;
-    Py_ssize_t n_zeros;
-    int loop_broken = 0;
-    int use_separator = 0; /* First time through, don't append the
-                              separator. They only go between
-                              groups. */
-    STRINGLIB_CHAR *buffer_end = NULL;
-    STRINGLIB_CHAR *digits_end = NULL;
-    Py_ssize_t l;
-    Py_ssize_t n_chars;
-    Py_ssize_t remaining = n_digits; /* Number of chars remaining to
-                                        be looked at */
-    /* A generator that returns all of the grouping widths, until it
-       returns 0. */
-    STRINGLIB(GroupGenerator) groupgen;
-    STRINGLIB(GroupGenerator_init)(&groupgen, grouping);
-
-    if (buffer) {
-        buffer_end = buffer + n_buffer;
-        digits_end = digits + n_digits;
-    }
-
-    while ((l = STRINGLIB(GroupGenerator_next)(&groupgen)) > 0) {
-        l = Py_MIN(l, Py_MAX(Py_MAX(remaining, min_width), 1));
-        n_zeros = Py_MAX(0, l - remaining);
-        n_chars = Py_MAX(0, Py_MIN(remaining, l));
-
-        /* Use n_zero zero's and n_chars chars */
-
-        /* Count only, don't do anything. */
-        count += (use_separator ? thousands_sep_len : 0) + n_zeros + n_chars;
-
-        if (buffer) {
-            /* Copy into the output buffer. */
-            STRINGLIB(fill)(&digits_end, &buffer_end, n_chars, n_zeros,
-                 use_separator ? thousands_sep : NULL, thousands_sep_len);
-        }
-
-        /* Use a separator next time. */
-        use_separator = 1;
-
-        remaining -= n_chars;
-        min_width -= l;
-
-        if (remaining <= 0 && min_width <= 0) {
-            loop_broken = 1;
-            break;
-        }
-        min_width -= thousands_sep_len;
-    }
-    if (!loop_broken) {
-        /* We left the loop without using a break statement. */
-
-        l = Py_MAX(Py_MAX(remaining, min_width), 1);
-        n_zeros = Py_MAX(0, l - remaining);
-        n_chars = Py_MAX(0, Py_MIN(remaining, l));
-
-        /* Use n_zero zero's and n_chars chars */
-        count += (use_separator ? thousands_sep_len : 0) + n_zeros + n_chars;
-        if (buffer) {
-            /* Copy into the output buffer. */
-            STRINGLIB(fill)(&digits_end, &buffer_end, n_chars, n_zeros,
-                 use_separator ? thousands_sep : NULL, thousands_sep_len);
-        }
-    }
-    return count;
-}
-
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 71eb654..8dd7c3b 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -218,6 +218,31 @@
         return unicode_empty;                           \
     } while (0)
 
+#define FILL(kind, data, value, start, length) \
+    do { \
+        Py_ssize_t i_ = 0; \
+        assert(kind != PyUnicode_WCHAR_KIND); \
+        switch ((kind)) { \
+        case PyUnicode_1BYTE_KIND: { \
+            unsigned char * to_ = (unsigned char *)((data)) + (start); \
+            memset(to_, (unsigned char)value, (length)); \
+            break; \
+        } \
+        case PyUnicode_2BYTE_KIND: { \
+            Py_UCS2 * to_ = (Py_UCS2 *)((data)) + (start); \
+            for (; i_ < (length); ++i_, ++to_) *to_ = (value); \
+            break; \
+        } \
+        case PyUnicode_4BYTE_KIND: { \
+            Py_UCS4 * to_ = (Py_UCS4 *)((data)) + (start); \
+            for (; i_ < (length); ++i_, ++to_) *to_ = (value); \
+            break; \
+        } \
+        default: Py_UNREACHABLE(); \
+        } \
+    } while (0)
+
+
 /* Forward declaration */
 static inline int
 _PyUnicodeWriter_WriteCharInline(_PyUnicodeWriter *writer, Py_UCS4 ch);
@@ -796,7 +821,6 @@
 #include "stringlib/count.h"
 #include "stringlib/find.h"
 #include "stringlib/find_max_char.h"
-#include "stringlib/localeutil.h"
 #include "stringlib/undef.h"
 
 #include "stringlib/ucs1lib.h"
@@ -807,7 +831,6 @@
 #include "stringlib/find.h"
 #include "stringlib/replace.h"
 #include "stringlib/find_max_char.h"
-#include "stringlib/localeutil.h"
 #include "stringlib/undef.h"
 
 #include "stringlib/ucs2lib.h"
@@ -818,7 +841,6 @@
 #include "stringlib/find.h"
 #include "stringlib/replace.h"
 #include "stringlib/find_max_char.h"
-#include "stringlib/localeutil.h"
 #include "stringlib/undef.h"
 
 #include "stringlib/ucs4lib.h"
@@ -829,7 +851,6 @@
 #include "stringlib/find.h"
 #include "stringlib/replace.h"
 #include "stringlib/find_max_char.h"
-#include "stringlib/localeutil.h"
 #include "stringlib/undef.h"
 
 #include "stringlib/unicodedefs.h"
@@ -9292,86 +9313,149 @@
     return result;
 }
 
+/* _PyUnicode_InsertThousandsGrouping() helper functions */
+#include "stringlib/localeutil.h"
+
+/**
+ * InsertThousandsGrouping:
+ * @writer: Unicode writer.
+ * @n_buffer: Number of characters in @buffer.
+ * @digits: Digits we're reading from. If count is non-NULL, this is unused.
+ * @d_pos: Start of digits string.
+ * @n_digits: The number of digits in the string, in which we want
+ *            to put the grouping chars.
+ * @min_width: The minimum width of the digits in the output string.
+ *             Output will be zero-padded on the left to fill.
+ * @grouping: see definition in localeconv().
+ * @thousands_sep: see definition in localeconv().
+ *
+ * There are 2 modes: counting and filling. If @writer is NULL,
+ *  we are in counting mode, else filling mode.
+ * If counting, the required buffer size is returned.
+ * If filling, we know the buffer will be large enough, so we don't
+ *  need to pass in the buffer size.
+ * Inserts thousand grouping characters (as defined by grouping and
+ *  thousands_sep) into @writer.
+ *
+ * Return value: -1 on error, number of characters otherwise.
+ **/
 Py_ssize_t
 _PyUnicode_InsertThousandsGrouping(
-    PyObject *unicode, Py_ssize_t index,
+    _PyUnicodeWriter *writer,
     Py_ssize_t n_buffer,
-    void *digits, Py_ssize_t n_digits,
+    PyObject *digits,
+    Py_ssize_t d_pos,
+    Py_ssize_t n_digits,
     Py_ssize_t min_width,
-    const char *grouping, PyObject *thousands_sep,
+    const char *grouping,
+    PyObject *thousands_sep,
     Py_UCS4 *maxchar)
 {
-    unsigned int kind, thousands_sep_kind;
-    char *data, *thousands_sep_data;
-    Py_ssize_t thousands_sep_len;
-    Py_ssize_t len;
-
-    if (unicode != NULL) {
-        kind = PyUnicode_KIND(unicode);
-        data = (char *) PyUnicode_DATA(unicode) + index * kind;
+    if (writer) {
+        assert(digits != NULL);
+        assert(maxchar == NULL);
     }
     else {
-        kind = PyUnicode_1BYTE_KIND;
-        data = NULL;
+        assert(digits == NULL);
+        assert(maxchar != NULL);
     }
-    thousands_sep_kind = PyUnicode_KIND(thousands_sep);
-    thousands_sep_data = PyUnicode_DATA(thousands_sep);
-    thousands_sep_len = PyUnicode_GET_LENGTH(thousands_sep);
-    if (unicode != NULL && thousands_sep_kind != kind) {
-        if (thousands_sep_kind < kind) {
-            thousands_sep_data = _PyUnicode_AsKind(thousands_sep, kind);
-            if (!thousands_sep_data)
-                return -1;
+    assert(0 <= d_pos);
+    assert(0 <= n_digits);
+    assert(0 <= min_width);
+    assert(grouping != NULL);
+
+    if (digits != NULL) {
+        if (PyUnicode_READY(digits) == -1) {
+            return -1;
         }
-        else {
-            data = _PyUnicode_AsKind(unicode, thousands_sep_kind);
-            if (!data)
-                return -1;
-        }
+    }
+    if (PyUnicode_READY(thousands_sep) == -1) {
+        return -1;
     }
 
-    switch (kind) {
-    case PyUnicode_1BYTE_KIND:
-        if (unicode != NULL && PyUnicode_IS_ASCII(unicode))
-            len = asciilib_InsertThousandsGrouping(
-                (Py_UCS1 *) data, n_buffer, (Py_UCS1 *) digits, n_digits,
-                min_width, grouping,
-                (Py_UCS1 *) thousands_sep_data, thousands_sep_len);
-        else
-            len = ucs1lib_InsertThousandsGrouping(
-                (Py_UCS1*)data, n_buffer, (Py_UCS1*)digits, n_digits,
-                min_width, grouping,
-                (Py_UCS1 *) thousands_sep_data, thousands_sep_len);
-        break;
-    case PyUnicode_2BYTE_KIND:
-        len = ucs2lib_InsertThousandsGrouping(
-            (Py_UCS2 *) data, n_buffer, (Py_UCS2 *) digits, n_digits,
-            min_width, grouping,
-            (Py_UCS2 *) thousands_sep_data, thousands_sep_len);
-        break;
-    case PyUnicode_4BYTE_KIND:
-        len = ucs4lib_InsertThousandsGrouping(
-            (Py_UCS4 *) data, n_buffer, (Py_UCS4 *) digits, n_digits,
-            min_width, grouping,
-            (Py_UCS4 *) thousands_sep_data, thousands_sep_len);
-        break;
-    default:
-        Py_UNREACHABLE();
+    Py_ssize_t count = 0;
+    Py_ssize_t n_zeros;
+    int loop_broken = 0;
+    int use_separator = 0; /* First time through, don't append the
+                              separator. They only go between
+                              groups. */
+    Py_ssize_t buffer_pos;
+    Py_ssize_t digits_pos;
+    Py_ssize_t len;
+    Py_ssize_t n_chars;
+    Py_ssize_t remaining = n_digits; /* Number of chars remaining to
+                                        be looked at */
+    /* A generator that returns all of the grouping widths, until it
+       returns 0. */
+    GroupGenerator groupgen;
+    GroupGenerator_init(&groupgen, grouping);
+    const Py_ssize_t thousands_sep_len = PyUnicode_GET_LENGTH(thousands_sep);
+
+    /* if digits are not grouped, thousands separator
+       should be an empty string */
+    assert(!(grouping[0] == CHAR_MAX && thousands_sep_len != 0));
+
+    digits_pos = d_pos + n_digits;
+    if (writer) {
+        buffer_pos = writer->pos + n_buffer;
+        assert(buffer_pos <= PyUnicode_GET_LENGTH(writer->buffer));
+        assert(digits_pos <= PyUnicode_GET_LENGTH(digits));
     }
-    if (unicode != NULL && thousands_sep_kind != kind) {
-        if (thousands_sep_kind < kind)
-            PyMem_Free(thousands_sep_data);
-        else
-            PyMem_Free(data);
+    else {
+        buffer_pos = n_buffer;
     }
-    if (unicode == NULL) {
+
+    if (!writer) {
         *maxchar = 127;
-        if (len != n_digits) {
-            *maxchar = Py_MAX(*maxchar,
-                                   PyUnicode_MAX_CHAR_VALUE(thousands_sep));
-        }
     }
-    return len;
+
+    while ((len = GroupGenerator_next(&groupgen)) > 0) {
+        len = Py_MIN(len, Py_MAX(Py_MAX(remaining, min_width), 1));
+        n_zeros = Py_MAX(0, len - remaining);
+        n_chars = Py_MAX(0, Py_MIN(remaining, len));
+
+        /* Use n_zero zero's and n_chars chars */
+
+        /* Count only, don't do anything. */
+        count += (use_separator ? thousands_sep_len : 0) + n_zeros + n_chars;
+
+        /* Copy into the writer. */
+        InsertThousandsGrouping_fill(writer, &buffer_pos,
+                                     digits, &digits_pos,
+                                     n_chars, n_zeros,
+                                     use_separator ? thousands_sep : NULL,
+                                     thousands_sep_len, maxchar);
+
+        /* Use a separator next time. */
+        use_separator = 1;
+
+        remaining -= n_chars;
+        min_width -= len;
+
+        if (remaining <= 0 && min_width <= 0) {
+            loop_broken = 1;
+            break;
+        }
+        min_width -= thousands_sep_len;
+    }
+    if (!loop_broken) {
+        /* We left the loop without using a break statement. */
+
+        len = Py_MAX(Py_MAX(remaining, min_width), 1);
+        n_zeros = Py_MAX(0, len - remaining);
+        n_chars = Py_MAX(0, Py_MIN(remaining, len));
+
+        /* Use n_zero zero's and n_chars chars */
+        count += (use_separator ? thousands_sep_len : 0) + n_zeros + n_chars;
+
+        /* Copy into the writer. */
+        InsertThousandsGrouping_fill(writer, &buffer_pos,
+                                     digits, &digits_pos,
+                                     n_chars, n_zeros,
+                                     use_separator ? thousands_sep : NULL,
+                                     thousands_sep_len, maxchar);
+    }
+    return count;
 }
 
 
@@ -9990,30 +10074,6 @@
     return NULL;
 }
 
-#define FILL(kind, data, value, start, length) \
-    do { \
-        Py_ssize_t i_ = 0; \
-        assert(kind != PyUnicode_WCHAR_KIND); \
-        switch ((kind)) { \
-        case PyUnicode_1BYTE_KIND: { \
-            unsigned char * to_ = (unsigned char *)((data)) + (start); \
-            memset(to_, (unsigned char)value, (length)); \
-            break; \
-        } \
-        case PyUnicode_2BYTE_KIND: { \
-            Py_UCS2 * to_ = (Py_UCS2 *)((data)) + (start); \
-            for (; i_ < (length); ++i_, ++to_) *to_ = (value); \
-            break; \
-        } \
-        case PyUnicode_4BYTE_KIND: { \
-            Py_UCS4 * to_ = (Py_UCS4 *)((data)) + (start); \
-            for (; i_ < (length); ++i_, ++to_) *to_ = (value); \
-            break; \
-        } \
-        default: Py_UNREACHABLE(); \
-        } \
-    } while (0)
-
 void
 _PyUnicode_FastFill(PyObject *unicode, Py_ssize_t start, Py_ssize_t length,
                     Py_UCS4 fill_char)
diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
index 2cd3eb8..ef81d15 100644
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -461,7 +461,8 @@
 /* not all fields of format are used.  for example, precision is
    unused.  should this take discrete params in order to be more clear
    about what it does?  or is passing a single format parameter easier
-   and more efficient enough to justify a little obfuscation? */
+   and more efficient enough to justify a little obfuscation?
+   Return -1 on error. */
 static Py_ssize_t
 calc_number_widths(NumberFieldWidths *spec, Py_ssize_t n_prefix,
                    Py_UCS4 sign_char, PyObject *number, Py_ssize_t n_start,
@@ -540,9 +541,12 @@
         Py_UCS4 grouping_maxchar;
         spec->n_grouped_digits = _PyUnicode_InsertThousandsGrouping(
             NULL, 0,
-            0, NULL,
-            spec->n_digits, spec->n_min_width,
+            NULL, 0, spec->n_digits,
+            spec->n_min_width,
             locale->grouping, locale->thousands_sep, &grouping_maxchar);
+        if (spec->n_grouped_digits == -1) {
+            return -1;
+        }
         *maxchar = Py_MAX(*maxchar, grouping_maxchar);
     }
 
@@ -634,26 +638,14 @@
     /* Only for type 'c' special case, it has no digits. */
     if (spec->n_digits != 0) {
         /* Fill the digits with InsertThousandsGrouping. */
-        char *pdigits;
-        if (PyUnicode_READY(digits))
-            return -1;
-        pdigits = PyUnicode_DATA(digits);
-        if (PyUnicode_KIND(digits) < kind) {
-            pdigits = _PyUnicode_AsKind(digits, kind);
-            if (pdigits == NULL)
-                return -1;
-        }
         r = _PyUnicode_InsertThousandsGrouping(
-                writer->buffer, writer->pos,
-                spec->n_grouped_digits,
-                pdigits + kind * d_pos,
-                spec->n_digits, spec->n_min_width,
+                writer, spec->n_grouped_digits,
+                digits, d_pos, spec->n_digits,
+                spec->n_min_width,
                 locale->grouping, locale->thousands_sep, NULL);
         if (r == -1)
             return -1;
         assert(r == spec->n_grouped_digits);
-        if (PyUnicode_KIND(digits) < kind)
-            PyMem_Free(pdigits);
         d_pos += spec->n_digits;
     }
     if (toupper) {
@@ -993,6 +985,9 @@
     n_total = calc_number_widths(&spec, n_prefix, sign_char, tmp, inumeric_chars,
                                  inumeric_chars + n_digits, n_remainder, 0,
                                  &locale, format, &maxchar);
+    if (n_total == -1) {
+        goto done;
+    }
 
     /* Allocate the memory. */
     if (_PyUnicodeWriter_Prepare(writer, n_total, maxchar) == -1)
@@ -1138,6 +1133,9 @@
     n_total = calc_number_widths(&spec, 0, sign_char, unicode_tmp, index,
                                  index + n_digits, n_remainder, has_decimal,
                                  &locale, format, &maxchar);
+    if (n_total == -1) {
+        goto done;
+    }
 
     /* Allocate the memory. */
     if (_PyUnicodeWriter_Prepare(writer, n_total, maxchar) == -1)
@@ -1321,6 +1319,9 @@
                                     i_re, i_re + n_re_digits, n_re_remainder,
                                     re_has_decimal, &locale, &tmp_format,
                                     &maxchar);
+    if (n_re_total == -1) {
+        goto done;
+    }
 
     /* Same formatting, but always include a sign, unless the real part is
      * going to be omitted, in which case we use whatever sign convention was
@@ -1331,6 +1332,9 @@
                                     i_im, i_im + n_im_digits, n_im_remainder,
                                     im_has_decimal, &locale, &tmp_format,
                                     &maxchar);
+    if (n_im_total == -1) {
+        goto done;
+    }
 
     if (skip_re)
         n_re_total = 0;