diff -r 8e67d5dc069d Lib/test/test_codecs.py
--- a/Lib/test/test_codecs.py	Fri Oct 26 17:05:55 2012 -0700
+++ b/Lib/test/test_codecs.py	Sat Oct 27 12:58:41 2012 +0300
@@ -666,8 +666,10 @@
         self.assertEqual(b"\xf0\x90\xbf\xbf\xed\xa0\x80".decode("utf-8", "surrogatepass"),
                          "\U00010fff\uD800")
         self.assertTrue(codecs.lookup_error("surrogatepass"))
-        with self.assertRaises(UnicodeDecodeError):
-            b"abc\xed\xa0".decode("utf-8", "surrogatepass")
+        self.assertRaises(UnicodeDecodeError, b"abc\xed\xa0".decode,
+                          "utf-8", "surrogatepass")
+        self.assertRaises(UnicodeDecodeError, b"abc\xed\xa0z".decode,
+                          "utf-8", "surrogatepass")
 
 @unittest.skipUnless(sys.platform == 'win32',
                      'cp65001 is a Windows-only codec')
diff -r 8e67d5dc069d Python/codecs.c
--- a/Python/codecs.c	Fri Oct 26 17:05:55 2012 -0700
+++ b/Python/codecs.c	Sat Oct 27 12:58:41 2012 +0300
@@ -791,10 +791,10 @@
         /* Try decoding a single surrogate character. If
            there are more, let the codec call us again. */
         p += start;
-        if (strlen(p) > 2 &&
-            ((p[0] & 0xf0) == 0xe0 ||
-             (p[1] & 0xc0) == 0x80 ||
-             (p[2] & 0xc0) == 0x80)) {
+        if (PyBytes_GET_SIZE(object) - start >= 3 &&
+            (p[0] & 0xf0) == 0xe0 &&
+            (p[1] & 0xc0) == 0x80 &&
+            (p[2] & 0xc0) == 0x80) {
             /* it's a three-byte code */
             ch = ((p[0] & 0x0f) << 12) + ((p[1] & 0x3f) << 6) + (p[2] & 0x3f);
             if (ch < 0xd800 || ch > 0xdfff)