diff --git a/src/rocker/volume_extension.py b/src/rocker/volume_extension.py
index 6f2626f..77cda95 100644
--- a/src/rocker/volume_extension.py
+++ b/src/rocker/volume_extension.py
@@ -61,7 +61,7 @@ def get_docker_args(self, cli_args):
         return ' '.join(args)
 
     @staticmethod
-    def register_arguments(parser):
+    def register_arguments(parser, default={}):
         parser.add_argument(Volume.ARG_ROCKER_VOLUME,
             metavar='HOST-DIR[:CONTAINER-DIR[:OPTIONS]]',
             type=str,