diff --git a/src/tests/mutter_dbusrunner.py b/src/tests/mutter_dbusrunner.py
index e4aa9b96d..7f0448c37 100644
--- a/src/tests/mutter_dbusrunner.py
+++ b/src/tests/mutter_dbusrunner.py
@@ -311,7 +311,7 @@ def meta_run(klass, extra_env=None, setup_argparse=None, handle_argparse=None):
 
     launch = os.getenv('MUTTER_DBUS_RUNNER_LAUNCH')
     if launch:
-        args.launch.append(launch.split(','))
+        args.launch += launch.split(',')
 
     if args.no_isolate_dirs:
         return meta_run_klass(klass, args, rest, extra_env)