diff --git a/core/admin/mailu/internal/nginx.py b/core/admin/mailu/internal/nginx.py
index 40d72505..b7e246af 100644
--- a/core/admin/mailu/internal/nginx.py
+++ b/core/admin/mailu/internal/nginx.py
@@ -94,7 +94,7 @@ def handle_authentication(headers):
         else:
             try:
                 user = models.User.query.get(user_email) if '@' in user_email else None
-                is_valid_user = bool(user)
+                is_valid_user = user is not None
             except sqlalchemy.exc.StatementError as exc:
                 exc = str(exc).split('\n', 1)[0]
                 app.logger.warn(f'Invalid user {user_email!r}: {exc}')