diff -r 7bff23ec3f5b Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py	Sun Jul 17 10:13:44 2011 -0400
+++ b/Lib/test/test_unicode.py	Sun Jul 17 20:59:03 2011 -0400
@@ -1,4 +1,4 @@
-""" Test script for the Unicode implementation.
+""" Test script forathe Unicode implementation.
 
 Written by Marc-Andre Lemburg (mal@lemburg.com).
 
@@ -736,6 +736,11 @@
         self.assertRaises(TypeError, '{a'.format_map)
         self.assertRaises(TypeError, '}a'.format_map)
 
+        # test positional error
+        self.assertRaises(ValueError, '{}'.format_map, {'a' : 2})
+        self.assertRaises(ValueError, '{}'.format_map, 'a')
+        self.assertRaises(ValueError, '{a} {}'.format_map, {"a" : 2, "b" : 1})
+
     def test_format_auto_numbering(self):
         class C:
             def __init__(self, x=100):
diff -r 7bff23ec3f5b Misc/ACKS
--- a/Misc/ACKS	Sun Jul 17 10:13:44 2011 -0400
+++ b/Misc/ACKS	Sun Jul 17 20:59:03 2011 -0400
@@ -82,6 +82,7 @@
 Andrew Bennetts
 Andy Bensky
 Michel Van den Bergh
+Julian Berman
 Eric Beser
 Steven Bethard
 Stephen Bevan
diff -r 7bff23ec3f5b Objects/stringlib/string_format.h
--- a/Objects/stringlib/string_format.h	Sun Jul 17 10:13:44 2011 -0400
+++ b/Objects/stringlib/string_format.h	Sun Jul 17 20:59:03 2011 -0400
@@ -511,6 +511,14 @@
         Py_DECREF(key);
     }
     else {
+        /* If args is NULL, we have a format string with a positional field
+           with only kwargs to retrieve it from. */
+        if (args == NULL) {
+            PyErr_SetString(PyExc_ValueError, "Format string contained "
+                            "positional fields");
+            goto error;
+        }
+
         /* look up in args */
         obj = PySequence_GetItem(args, index);
         if (obj == NULL)