diff --git a/winsup/cygwin/fhandler/proc.cc b/winsup/cygwin/fhandler/proc.cc
index be107cb8ea..cf40e8262c 100644
--- a/winsup/cygwin/fhandler/proc.cc
+++ b/winsup/cygwin/fhandler/proc.cc
@@ -2153,7 +2153,7 @@ format_proc_locale_proc (LPWSTR win_locale, DWORD info, LPARAM param)
       if (!(cp2 = wcschr (cp + 2, L'-')))
         return TRUE;
       /* Otherwise, store in iso15924 */
-      if (iso15924)
+      if (cp2 - cp + 1 < ENCODING_LEN)
         wcpcpy (wcpncpy (iso15924, cp, cp2 - cp), L";");
     }
   cp = wcsrchr (win_locale, L'-');