diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 4ae7377..1375ef3 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -2983,6 +2983,8 @@
     char *l_end;
 
     if (encoding == NULL) {
+        if (lower_len < 6)
+            return 0;
         strcpy(lower, "utf-8");
         return 1;
     }