diff --git a/Lib/argparse.py b/Lib/argparse.py
index 75cc28a..8e1722a 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1482,6 +1482,7 @@
         self._defaults = container._defaults
         self._has_negative_number_optionals = \
             container._has_negative_number_optionals
+        self._mutually_exclusive_groups = container._mutually_exclusive_groups
 
     def _add_action(self, action):
         action = super(_ArgumentGroup, self)._add_action(action)