diff --git a/Lib/argparse.py b/Lib/argparse.py
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -2211,7 +2211,8 @@
                 value = action.default
             else:
                 value = arg_strings
-            self._check_value(action, value)
+            for v in value:
+                self._check_value(action, v)
 
         # single argument or optional argument produces a single value
         elif len(arg_strings) == 1 and action.nargs in [None, OPTIONAL]: