diff --git a/Objects/stringlib/count.h b/Objects/stringlib/count.h
index 0bd02b5..84a852f 100644
--- a/Objects/stringlib/count.h
+++ b/Objects/stringlib/count.h
@@ -13,8 +13,11 @@
 {
     Py_ssize_t count;
 
-    if (sub_len == 0)
+    if (sub_len == 0) {
+        if (str_len < 0)
+            return 1; /* start >= len(str) */
         return str_len + 1;
+    }
 
     count = fastsearch(str, str_len, sub, sub_len, FAST_COUNT);