diff --git a/Objects/stringlib/localeutil.h b/Objects/stringlib/localeutil.h
index 28c87c8..6e2f073 100644
--- a/Objects/stringlib/localeutil.h
+++ b/Objects/stringlib/localeutil.h
@@ -99,7 +99,7 @@
  * As closely as possible, this code mimics the logic in decimal.py's
     _insert_thousands_sep().
  **/
-Py_ssize_t
+static Py_ssize_t
 STRINGLIB(InsertThousandsGrouping)(
     STRINGLIB_CHAR *buffer,
     Py_ssize_t n_buffer,