diff --git a/zerver/tests/test_service_bot_system.py b/zerver/tests/test_service_bot_system.py
index 6978640246..eba9854448 100644
--- a/zerver/tests/test_service_bot_system.py
+++ b/zerver/tests/test_service_bot_system.py
@@ -299,7 +299,7 @@ class TestServiceBotStateHandler(ZulipTestCase):
             'storage': ujson.dumps({'foo': [1, 2, 3]}),
         }
         result = self.client_put('/json/bot_storage', params)
-        self.assert_json_error(result, "Value type is <class 'list'>, but should be str.")
+        self.assert_json_error(result, "storage contains a value that is not a string")
 
         # Remove some entries.
         keys_to_remove = ['key 1', 'key 2']
diff --git a/zerver/views/storage.py b/zerver/views/storage.py
index 3772345cc0..1a8e959531 100644
--- a/zerver/views/storage.py
+++ b/zerver/views/storage.py
@@ -16,11 +16,13 @@ from zerver.models import UserProfile
 
 
 @has_request_variables
-def update_storage(request: HttpRequest, user_profile: UserProfile,
-                   storage: Dict[str, str]=REQ(validator=check_dict([]))) -> HttpResponse:
+def update_storage(
+    request: HttpRequest, user_profile: UserProfile,
+    storage: Dict[str, str]=REQ(validator=check_dict([], value_validator=check_string)),
+) -> HttpResponse:
     try:
         set_bot_storage(user_profile, list(storage.items()))
-    except StateError as e:
+    except StateError as e:  # nocoverage
         return json_error(str(e))
     return json_success()