diff --git a/src/rocker/core.py b/src/rocker/core.py
index e9a3c242..bf46abf2 100644
--- a/src/rocker/core.py
+++ b/src/rocker/core.py
@@ -117,7 +117,9 @@ def check_args_for_activation(cls, cli_args):
         return True if cli_args.get(cls.get_name()) else False
 
     @staticmethod
-    def register_arguments(parser, defaults={}):
+    def register_arguments(parser, defaults:dict=None):
+        if defaults is None:
+            defaults={}
         raise NotImplementedError