diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 35aa79f..9ab366d 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -1637,6 +1637,51 @@
     return 0;
 }
 
+/* Copy a ASCII or latin1 char* string into a Python Unicode string.
+   Return the length of the input string.
+
+   WARNING: Don't copy the terminating null character and don't check the
+   maximum character (may write a latin1 character in an ASCII string). */
+static Py_ssize_t
+unicode_write_cstr(PyObject *unicode, Py_ssize_t index, const char *str)
+{
+    enum PyUnicode_Kind kind = PyUnicode_KIND(unicode);
+    void *data = PyUnicode_DATA(unicode);
+
+    switch (kind) {
+    case PyUnicode_1BYTE_KIND: {
+        Py_ssize_t len = strlen(str);
+        assert(index + len <= PyUnicode_GET_LENGTH(unicode));
+        memcpy(data + index, str, len);
+        return len;
+    }
+    case PyUnicode_2BYTE_KIND: {
+        Py_UCS2 *start = (Py_UCS2 *)data + index;
+        Py_UCS2 *ucs2 = start;
+        assert(index <= PyUnicode_GET_LENGTH(unicode));
+
+        for (; *str; ++ucs2, ++str)
+            *ucs2 = (Py_UCS2)*str;
+
+        assert((ucs2 - start) <= PyUnicode_GET_LENGTH(unicode));
+        return ucs2 - start;
+    }
+    default: {
+        Py_UCS4 *start = (Py_UCS4 *)data + index;
+        Py_UCS4 *ucs4 = start;
+        assert(kind == PyUnicode_4BYTE_KIND);
+        assert(index <= PyUnicode_GET_LENGTH(unicode));
+
+        for (; *str; ++ucs4, ++str)
+            *ucs4 = (Py_UCS4)*str;
+
+        assert((ucs4 - start) <= PyUnicode_GET_LENGTH(unicode));
+        return ucs4 - start;
+    }
+    }
+}
+
+
 static PyObject*
 get_latin1_char(unsigned char ch)
 {
@@ -2590,19 +2635,23 @@
             case 'u':
             case 'x':
             case 'p':
+            {
+                Py_ssize_t written;
                 /* unused, since we already have the result */
                 if (*f == 'p')
                     (void) va_arg(vargs, void *);
                 else
                     (void) va_arg(vargs, int);
                 /* extract the result from numberresults and append. */
-                for (; *numberresult; ++i, ++numberresult)
-                    PyUnicode_WRITE(kind, data, i, *numberresult);
+                written = unicode_write_cstr(string, i, numberresult);
                 /* skip over the separating '\0' */
+                i += written;
+                numberresult += written;
                 assert(*numberresult == '\0');
                 numberresult++;
                 assert(numberresult <= numberresults + numbersize);
                 break;
+            }
             case 's':
             {
                 /* unused, since we already have the result */
@@ -2669,8 +2718,7 @@
                 PyUnicode_WRITE(kind, data, i++, '%');
                 break;
             default:
-                for (; *p; ++p, ++i)
-                    PyUnicode_WRITE(kind, data, i, *p);
+                i += unicode_write_cstr(string, i, p);
                 assert(i == PyUnicode_GET_LENGTH(string));
                 goto end;
             }