diff --git a/zerver/lib/html_diff.py b/zerver/lib/html_diff.py
index 854374a101..eba1047602 100644
--- a/zerver/lib/html_diff.py
+++ b/zerver/lib/html_diff.py
@@ -1,5 +1,5 @@
 
-from typing import Callable, List, Tuple, Text
+from typing import Callable, List, Optional, Tuple, Text
 
 from django.conf import settings