diff --git a/changelog.d/14637.bugfix b/changelog.d/14637.bugfix
new file mode 100644
index 0000000000..ab6db383c6
--- /dev/null
+++ b/changelog.d/14637.bugfix
@@ -0,0 +1 @@
+Fix a bug introduced in v1.67.0 where not specifying a config file or a server URL would lead to the `register_new_matrix_user` script failing.
\ No newline at end of file
diff --git a/synapse/_scripts/register_new_matrix_user.py b/synapse/_scripts/register_new_matrix_user.py
index 0c4504d5d8..2b74a40166 100644
--- a/synapse/_scripts/register_new_matrix_user.py
+++ b/synapse/_scripts/register_new_matrix_user.py
@@ -222,6 +222,7 @@ def main() -> None:
 
     args = parser.parse_args()
 
+    config: Optional[Dict[str, Any]] = None
     if "config" in args and args.config:
         config = yaml.safe_load(args.config)
 
@@ -229,7 +230,7 @@ def main() -> None:
         secret = args.shared_secret
     else:
         # argparse should check that we have either config or shared secret
-        assert config
+        assert config is not None
 
         secret = config.get("registration_shared_secret")
         secret_file = config.get("registration_shared_secret_path")
@@ -244,7 +245,7 @@ def main() -> None:
 
     if args.server_url:
         server_url = args.server_url
-    elif config:
+    elif config is not None:
         server_url = _find_client_listener(config)
         if not server_url:
             server_url = _DEFAULT_SERVER_URL