diff --git a/zerver/context_processors.py b/zerver/context_processors.py
index 7aba55e633..4c8f6db340 100644
--- a/zerver/context_processors.py
+++ b/zerver/context_processors.py
@@ -11,6 +11,7 @@ from version import (
     ZULIP_VERSION,
 )
 from zerver.decorator import get_client_name
+from zerver.lib.exceptions import InvalidSubdomainError
 from zerver.lib.realm_description import get_realm_rendered_description, get_realm_text_description
 from zerver.lib.realm_icon import get_realm_icon_url
 from zerver.lib.send_email import FromAddress
@@ -54,6 +55,12 @@ def get_realm_from_request(request: HttpRequest) -> Optional[Realm]:
             request.realm = None
     return request.realm
 
+def get_valid_realm_from_request(request: HttpRequest) -> Realm:
+    realm = get_realm_from_request(request)
+    if realm is None:
+        raise InvalidSubdomainError()
+    return realm
+
 def zulip_default_context(request: HttpRequest) -> Dict[str, Any]:
     """Context available to all Zulip Jinja2 templates that have a request
     passed in.  Designed to provide the long list of variables at the
diff --git a/zerver/lib/exceptions.py b/zerver/lib/exceptions.py
index 397eaf6019..c1cad3d84a 100644
--- a/zerver/lib/exceptions.py
+++ b/zerver/lib/exceptions.py
@@ -46,6 +46,7 @@ class ErrorCode(AbstractEnum):
     INVALID_API_KEY = ()
     INVALID_ZOOM_TOKEN = ()
     UNAUTHENTICATED_USER = ()
+    NONEXISTENT_SUBDOMAIN = ()
 
 class JsonableError(Exception):
     '''A standardized error format we can turn into a nice JSON HTTP response.
@@ -277,3 +278,14 @@ class MissingAuthenticationError(JsonableError):
 
     # No msg_format is defined since this exception is caught and
     # converted into json_unauthorized in Zulip's middleware.
+
+class InvalidSubdomainError(JsonableError):
+    code = ErrorCode.NONEXISTENT_SUBDOMAIN
+    http_status_code = 404
+
+    def __init__(self) -> None:
+        pass
+
+    @staticmethod
+    def msg_format() -> str:
+        return _("Invalid subdomain")
diff --git a/zerver/tests/test_message_fetch.py b/zerver/tests/test_message_fetch.py
index 2eb7924ab3..52c1f506ed 100644
--- a/zerver/tests/test_message_fetch.py
+++ b/zerver/tests/test_message_fetch.py
@@ -1212,10 +1212,10 @@ class GetOldMessagesTest(ZulipTestCase):
             "narrow": orjson.dumps([dict(operator='streams', operand="web-public")]).decode(),
         }
 
-        with mock.patch('zerver.views.message_fetch.get_realm_from_request', return_value=None):
+        with mock.patch('zerver.context_processors.get_realm', side_effect=Realm.DoesNotExist):
             result = self.client_get("/json/messages", dict(post_params))
-            self.assert_json_error(result, "Invalid subdomain.",
-                                   status_code=400)
+            self.assert_json_error(result, "Invalid subdomain",
+                                   status_code=404)
 
     def test_unauthenticated_get_messages_without_web_public(self) -> None:
         """
diff --git a/zerver/views/message_fetch.py b/zerver/views/message_fetch.py
index 37ad48897d..713960f0ea 100644
--- a/zerver/views/message_fetch.py
+++ b/zerver/views/message_fetch.py
@@ -27,7 +27,7 @@ from sqlalchemy.sql import (
     union_all,
 )
 
-from zerver.context_processors import get_realm_from_request
+from zerver.context_processors import get_valid_realm_from_request
 from zerver.decorator import REQ, has_request_variables
 from zerver.lib.actions import recipient_for_user_profiles
 from zerver.lib.addressee import get_user_profiles, get_user_profiles_by_ids
@@ -870,10 +870,7 @@ def get_messages_backend(request: HttpRequest,
         if not is_web_public_compatible(narrow):
             return json_unauthorized()
 
-        realm = get_realm_from_request(request)
-        if realm is None:
-            return json_error(_("Invalid subdomain."))
-
+        realm = get_valid_realm_from_request(request)
         # We use None to indicate unauthenticated requests as it's more
         # readable than using AnonymousUser, and the lack of Django
         # stubs means that mypy can't check AnonymousUser well.