diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 775bd15..3d9e09d 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -4190,7 +4190,10 @@
     Py_ssize_t insize;
     Py_ssize_t newpos;
     Py_ssize_t replen;
+    Py_ssize_t remain;
     PyObject *inputobj = NULL;
+    int need_to_grow = 0;
+    const char *new_inptr;
 
     if (*errorHandler == NULL) {
         *errorHandler = PyCodec_LookupError(errors);
@@ -4221,6 +4224,7 @@
     inputobj = PyUnicodeDecodeError_GetObject(*exceptionObject);
     if (!inputobj)
         goto onError;
+    remain = *inend - *input - *endinpos;
     *input = PyBytes_AS_STRING(inputobj);
     insize = PyBytes_GET_SIZE(inputobj);
     *inend = *input + insize;
@@ -4238,6 +4242,19 @@
     replen = PyUnicode_GET_LENGTH(repunicode);
     if (replen > 1) {
         writer->min_length += replen - 1;
+        need_to_grow = 1;
+    }
+    new_inptr = *input + newpos;
+    if (*inend - new_inptr > remain) {
+        /* We don't know the decoding algorithm here so we make the worst
+           assumption that one byte decodes to one unicode character.
+           If unfortunately one byte could decode to more unicode characters,
+           the decoder may write out-of-bound then.  Is it possible for the
+           algorithms using this function? */
+        writer->min_length += *inend - new_inptr - remain;
+        need_to_grow = 1;
+    }
+    if (need_to_grow) {
         writer->overallocate = 1;
         if (_PyUnicodeWriter_Prepare(writer, writer->min_length,
                             PyUnicode_MAX_CHAR_VALUE(repunicode)) == -1)
@@ -4247,7 +4264,7 @@
         goto onError;
 
     *endinpos = newpos;
-    *inptr = *input + newpos;
+    *inptr = new_inptr;
 
     /* we made it! */
     Py_DECREF(restuple);
@@ -5572,7 +5589,8 @@
 #endif
 
     /* Note: size will always be longer than the resulting Unicode
-       character count */
+       character count normally.  Error handler will take care of
+       resizing when needed. */
     _PyUnicodeWriter_Init(&writer);
     writer.min_length = (e - q + 1) / 2;
     if (_PyUnicodeWriter_Prepare(&writer, writer.min_length, 127) == -1)