diff --git a/Misc/NEWS b/Misc/NEWS
index 4e4b47c7c06..6786cad716a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -200,6 +200,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #12404: Remove C89 incompatible code from mmap module. Patch by Akira
+  Kitada.
+
 - Issue #1874: email now detects and reports as a defect the presence of
   any CTE other than 7bit, 8bit, or binary on a multipart.
 
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index ab12e2c54c5..5d086a7386a 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -1162,12 +1162,13 @@ new_mmap_object(PyTypeObject *type, PyObject *args, PyObject *kwdict)
 #  endif
     if (fd != -1 && fstat(fd, &st) == 0 && S_ISREG(st.st_mode)) {
         if (map_size == 0) {
+            off_t calc_size;
             if (offset >= st.st_size) {
                 PyErr_SetString(PyExc_ValueError,
                                 "mmap offset is greater than file size");
                 return NULL;
             }
-            off_t calc_size = st.st_size - offset;
+            calc_size = st.st_size - offset;
             map_size = calc_size;
             if (map_size != calc_size) {
                 PyErr_SetString(PyExc_ValueError,