diff -r 8630fa732cf6 Lib/argparse.py
--- a/Lib/argparse.py	Thu Jan 17 17:07:17 2013 +0100
+++ b/Lib/argparse.py	Thu Jan 17 21:27:42 2013 +0100
@@ -581,9 +581,11 @@
             result = '...'
         elif action.nargs == PARSER:
             result = '%s ...' % get_metavar(1)
-        else:
+        elif isinstance(action.nargs, int):
             formats = ['%s' for _ in range(action.nargs)]
             result = ' '.join(formats) % get_metavar(action.nargs)
+        else:
+            raise ValueError("invalid value for nargs")
         return result
 
     def _expand_help(self, action):
@@ -833,7 +835,7 @@
                  required=False,
                  help=None,
                  metavar=None):
-        if nargs == 0:
+        if isinstance(nargs, int) and nargs <= 0:
             raise ValueError('nargs for store actions must be > 0; if you '
                              'have nothing to store, actions such as store '
                              'true or store const may be more appropriate')
@@ -925,7 +927,7 @@
                  required=False,
                  help=None,
                  metavar=None):
-        if nargs == 0:
+        if isinstance(nargs, int) and nargs <= 0:
             raise ValueError('nargs for append actions must be > 0; if arg '
                              'strings are not supplying the value to append, '
                              'the append const action may be more appropriate')
diff -r 8630fa732cf6 Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py	Thu Jan 17 17:07:17 2013 +0100
+++ b/Lib/test/test_argparse.py	Thu Jan 17 21:27:42 2013 +0100
@@ -4089,7 +4089,6 @@
 
 
 class TestHelpMetavarTypeFormatter(HelpTestCase):
-    """"""
 
     def custom_type(string):
         return string
@@ -4198,9 +4197,11 @@
     def test_more_than_one_argument_actions(self):
         for action in ['store', 'append']:
 
-            # nargs=0 is disallowed
+            # nargs<=0 is disallowed
             self.assertValueError('-x', nargs=0, action=action)
             self.assertValueError('spam', nargs=0, action=action)
+            self.assertValueError('-x', nargs=-1, action=action)
+            self.assertValueError('spam', nargs=-1, action=action)
 
             # const is disallowed with non-optional arguments
             for nargs in [1, '*', '+']:
@@ -4885,6 +4886,38 @@
     def test_nargs_3_metavar_length3(self):
         self.do_test_no_exception(nargs=3, metavar=("1", "2", "3"))
 
+
+class TestInvalidNargs(TestCase):
+
+    EXPECTED_INVALID_MESSAGE = "invalid value for nargs"
+    EXPECTED_RANGE_MESSAGE = "nargs for store actions must be > 0; if you " \
+                             "have nothing to store, actions such as store " \
+                             "true or store const may be more appropriate"
+
+    def do_test_range_exception(self, nargs):
+        parser = argparse.ArgumentParser()
+        with self.assertRaises(ValueError) as cm:
+            parser.add_argument("--foo", nargs=nargs)
+        self.assertEqual(cm.exception.args[0], self.EXPECTED_RANGE_MESSAGE)
+
+    def do_test_invalid_exception(self, nargs):
+        parser = argparse.ArgumentParser()
+        with self.assertRaises(ValueError) as cm:
+            parser.add_argument("--foo", nargs=nargs)
+        self.assertEqual(cm.exception.args[0], self.EXPECTED_INVALID_MESSAGE)
+
+    # Unit tests for different values of nargs
+
+    def test_nargs_alphabetic(self):
+        self.do_test_invalid_exception(nargs='a')
+        self.do_test_invalid_exception(nargs="abcd")
+
+    def test_nargs_zero(self):
+        self.do_test_range_exception(nargs=0)
+
+    def test_nargs_negative(self):
+        self.do_test_range_exception(nargs=-1)
+
 # ============================
 # from argparse import * tests
 # ============================