diff -r befd752a3218 Lib/argparse.py
--- a/Lib/argparse.py	Tue Sep 11 22:16:58 2012 -0700
+++ b/Lib/argparse.py	Wed Sep 12 11:42:33 2012 -0400
@@ -1948,7 +1948,7 @@
                     # twice (which may fail) if the argument was given, but
                     # only if it was defined already in the namespace
                     if (action.default is not None and
-                            isinstance(action, _StoreAction) and
+                            isinstance(action.default, str) and
                             hasattr(namespace, action.dest) and
                             action.default is getattr(namespace, action.dest)):
                         setattr(namespace, action.dest,
diff -r befd752a3218 Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py	Tue Sep 11 22:16:58 2012 -0700
+++ b/Lib/test/test_argparse.py	Wed Sep 12 11:42:33 2012 -0400
@@ -4478,7 +4478,8 @@
         parser = argparse.ArgumentParser()
         parser.add_argument('--foo', type=spam, default=0)
         args = parser.parse_args([])
-        self.assertEqual(NS(foo='foo_converted'), args)
+        # foo should *not* be converted because its default is not a string.
+        self.assertEqual(NS(foo=0), args)
 
     def test_issue_15906(self):
         # Issue #15906: When action='append', type=str, default=[] are