diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py
index 489af20..5fe9fb9 100644
--- a/Lib/test/string_tests.py
+++ b/Lib/test/string_tests.py
@@ -106,10 +106,18 @@
         self.checkequal(3, 'aaa', 'count', 'a')
         self.checkequal(0, 'aaa', 'count', 'b')
         self.checkequal(0, 'aaa', 'count', 'b')
+        self.checkequal(2, 'aaa', 'count', 'a', 1)
+        self.checkequal(0, 'aaa', 'count', 'a', 10)
         self.checkequal(1, 'aaa', 'count', 'a', -1)
         self.checkequal(3, 'aaa', 'count', 'a', -10)
+        self.checkequal(1, 'aaa', 'count', 'a', 0, 1)
+        self.checkequal(3, 'aaa', 'count', 'a', 0, 10)
         self.checkequal(2, 'aaa', 'count', 'a', 0, -1)
         self.checkequal(0, 'aaa', 'count', 'a', 0, -10)
+        self.checkequal(3, 'aaa', 'count', '', 1)
+        self.checkequal(1, 'aaa', 'count', '', 10)
+        self.checkequal(2, 'aaa', 'count', '', -1)
+        self.checkequal(4, 'aaa', 'count', '', -10)
 
         self.checkraises(TypeError, 'hello', 'count')
         self.checkraises(TypeError, 'hello', 'count', 42)
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);