diff --git a/endpoints/web.py b/endpoints/web.py
index be22dfeb7..cdb463cfa 100644
--- a/endpoints/web.py
+++ b/endpoints/web.py
@@ -843,6 +843,10 @@ def redirect_to_repository(namespace_name, repo_name, tag_name):
 @process_oauth
 @anon_protect
 def redirect_to_namespace(namespace):
+  okay, _ = model.user.validate_username(namespace)
+  if not okay:
+    abort(404)
+
   user_or_org = model.user.get_user_or_org(namespace)
   if not user_or_org:
     abort(404)