diff --git a/zerver/views/events_register.py b/zerver/views/events_register.py
index b91e44629e..dee9074b57 100644
--- a/zerver/views/events_register.py
+++ b/zerver/views/events_register.py
@@ -41,7 +41,7 @@ def events_register_backend(
             # Any new fields of `client_capabilities` should be optional. Add them here.
             ("bulk_message_deletion", check_bool),
             ('user_avatar_url_field_optional', check_bool),
-        ]), default=None),
+        ], value_validator=check_bool), default=None),
         event_types: Optional[Iterable[str]]=REQ(validator=check_list(check_string), default=None),
         fetch_event_types: Optional[Iterable[str]]=REQ(validator=check_list(check_string), default=None),
         narrow: NarrowT=REQ(validator=check_list(check_list(check_string, length=2)), default=[]),