diff -r 955547e57cff Lib/shutil.py
--- a/Lib/shutil.py	Sun Mar 13 19:32:21 2011 +0100
+++ b/Lib/shutil.py	Mon Mar 14 23:22:20 2011 -0400
@@ -737,8 +737,8 @@
         except KeyError:
             raise ValueError("Unknown unpack format '{0}'".format(format))
 
-        func = format_info[0]
-        func(filename, extract_dir, **dict(format_info[1]))
+        func = format_info[1]
+        func(filename, extract_dir, **dict(format_info[2]))
     else:
         # we need to look at the registered unpackers supported extensions
         format = _find_unpack_format(filename)
diff -r 955547e57cff Lib/test/test_shutil.py
--- a/Lib/test/test_shutil.py	Sun Mar 13 19:32:21 2011 +0100
+++ b/Lib/test/test_shutil.py	Mon Mar 14 23:22:20 2011 -0400
@@ -644,6 +644,14 @@
             unpack_archive(filename, tmpdir2)
             diff = self._compare_dirs(tmpdir, tmpdir2)
             self.assertEqual(diff, [])
+            shutil.rmtree(tmpdir2)
+
+            # and again, this time with the format specified
+            unpack_archive(filename, tmpdir2, format=format)
+            diff = self._compare_dirs(tmpdir, tmpdir2)
+            self.assertEqual(diff, [])
+        self.assertRaises(shutil.ReadError, unpack_archive, TESTFN)
+        self.assertRaises(ValueError, unpack_archive, TESTFN, format='xxx')
 
     def test_unpack_registery(self):