diff --git a/zerver/lib/types.py b/zerver/lib/types.py
index ad1b08724b..bb62c58509 100644
--- a/zerver/lib/types.py
+++ b/zerver/lib/types.py
@@ -1,4 +1,8 @@
-from typing import TypeVar, Callable
+from typing import TypeVar, Callable, Optional
 from django.http import HttpResponse
 
 ViewFuncT = TypeVar('ViewFuncT', bound=Callable[..., HttpResponse])
+
+# See zerver/lib/validator.py for more details of Validators,
+# including many examples
+Validator = Callable[[str, object], Optional[str]]