diff --git a/colcon_defaults/argument_parser/defaults.py b/colcon_defaults/argument_parser/defaults.py
index 062f3ae..f321849 100644
--- a/colcon_defaults/argument_parser/defaults.py
+++ b/colcon_defaults/argument_parser/defaults.py
@@ -108,6 +108,8 @@ def add_argument(self, *args, **kwargs):
             type_ = int
         elif kwargs.get('action') in ('store_false', 'store_true'):
             type_ = bool
+        elif callable(kwargs.get('action')):
+            type_ = None
         elif 'type' not in kwargs:
             type_ = str
         if kwargs.get('nargs') in ('*', '+'):