diff -r 44f455e6163d Lib/argparse.py
--- a/Lib/argparse.py	Thu Jun 27 12:23:29 2013 +0200
+++ b/Lib/argparse.py	Sat Apr 26 23:06:24 2014 -0700
@@ -530,9 +530,12 @@
     def _format_action_invocation(self, action):
         if not action.option_strings:
             default = self._get_default_metavar_for_positional(action)
-            metavar, = self._metavar_formatter(action, default)(1)
-            return metavar
-
+            metavar = self._metavar_formatter(action, default)(1)
+            if len(metavar)==1:
+                return metavar[0] # usual case
+            else:
+                # positional with tuple metavar needs special handling
+                return _format_metavars(action, self)
         else:
             parts = []
 
@@ -687,13 +690,30 @@
 # Options and Arguments
 # =====================
 
+def _format_metavars(action, formatter=None):
+    # issue14074 - for positional, turn a tuple metavar into a
+    # string that can be used for both help and error messages
+    # some alternative versions
+    return '|'.join(action.metavar)
+    #return str(action.metavar)
+    #return action.metavar[0]
+    if formatter is None:
+        # if called from a module function
+        formatter = HelpFormatter('')
+    # use format used in the usage line
+    metastr = formatter._format_args(action, action.dest)
+    return metastr
+
 def _get_action_name(argument):
     if argument is None:
         return None
     elif argument.option_strings:
         return  '/'.join(argument.option_strings)
     elif argument.metavar not in (None, SUPPRESS):
-        return argument.metavar
+        metavar = argument.metavar
+        if isinstance(metavar, tuple):
+            metavar = _format_metavars(argument)
+        return metavar
     elif argument.dest not in (None, SUPPRESS):
         return argument.dest
     else:
diff -r 44f455e6163d Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py	Thu Jun 27 12:23:29 2013 +0200
+++ b/Lib/test/test_argparse.py	Sat Apr 26 23:06:24 2014 -0700
@@ -3859,6 +3859,32 @@
         '''
     version = ''
 
+class TestHelpPositionalTupleMetavar(HelpTestCase):
+    """Test specifying metavar of a positional as a tuple"""
+
+    parser_signature = Sig(prog='PROG')
+    argument_signatures = [
+        Sig('w', help='w', nargs='+', metavar=('W1', 'W2')),
+        Sig('x', help='x', nargs='*', metavar=('X1', 'X2')),
+        Sig('y', help='y', nargs=3, metavar=('Y1', 'Y2', 'Y3')),
+        Sig('z', help='z', nargs='?', metavar=('Z1', )),
+    ]
+    argument_group_signatures = []
+    usage = '''\
+        usage: PROG [-h] W1 [W2 ...] [X1 [X2 ...]] Y1 Y2 Y3 [Z1]
+        '''
+    help = usage + '''\
+
+        positional arguments:
+          W1|W2       w
+          X1|X2       x
+          Y1|Y2|Y3    y
+          Z1          z
+
+        optional arguments:
+          -h, --help  show this help message and exit
+        '''
+    version = ''
 
 class TestHelpRawText(HelpTestCase):
     """Test the RawTextHelpFormatter"""
@@ -4606,6 +4632,22 @@
         self.assertRegex(msg, 'req_pos')
         self.assertNotIn(msg, 'optional_positional')
 
+    def test_positional_metavars(self):
+        """issue 14074 -
+        bad error message: TypeError: sequence item 0: expected str instance, tuple found
+        that is, it could not convert a tuple metavar as a string
+        test that proper 'arguments are required' error
+        """
+        parser = ErrorRaisingArgumentParser(prog='PROG', usage='')
+        parser.add_argument('pos', nargs=2, metavar=('A', 'B'))
+        parser.add_argument('star', nargs='*', metavar=('A', 'B'))
+        with self.assertRaises(ArgumentParserError) as cm:
+            parser.parse_args([])
+        msg = str(cm.exception)
+        self.assertRegex(msg, 'the following arguments are required')
+        #self.assertRegex(msg, 'A B')
+        #self.assertRegex(msg, r'\[A \[B \.\.\.\]\]')
+        self.assertRegex(msg, r'A\|B')
 
 # ================================================
 # Check that the type function is called only once