diff --git a/zerver/tornado/views.py b/zerver/tornado/views.py
index 40105d3f77..5bc7f0a5f7 100644
--- a/zerver/tornado/views.py
+++ b/zerver/tornado/views.py
@@ -72,7 +72,7 @@ def get_events_backend(request: HttpRequest, user_profile: UserProfile,
                        event_types: Optional[Sequence[str]]=REQ(default=None, validator=check_list(check_string),
                                                                 intentionally_undocumented=True),
                        dont_block: bool=REQ(default=False, validator=check_bool),
-                       narrow: Iterable[Sequence[str]]=REQ(default=[], validator=check_list(None),
+                       narrow: Iterable[Sequence[str]]=REQ(default=[], validator=check_list(check_list(check_string)),
                                                            intentionally_undocumented=True),
                        lifespan_secs: int=REQ(default=0, converter=to_non_negative_int,
                                               intentionally_undocumented=True),