diff -r d170844a363f Lib/test/test_ucn.py
--- a/Lib/test/test_ucn.py	Fri Nov 09 01:09:27 2012 +0200
+++ b/Lib/test/test_ucn.py	Fri Nov 09 16:36:22 2012 +0200
@@ -9,6 +9,7 @@
 
 import unittest
 import unicodedata
+import _testcapi
 
 from test import support
 from http.client import HTTPException
@@ -215,6 +216,20 @@
             str, b"\\NSPACE", 'unicode-escape', 'strict'
         )
 
+    @unittest.skipUnless(_testcapi.INT_MAX < _testcapi.PY_SSIZE_T_MAX,
+                         "needs UINT_MAX < SIZE_MAX")
+    @support.bigmemtest(size=_testcapi.UINT_MAX + 1 + len(b'\\N{SPACE}') + 1,
+                        memuse=1 + 1, dry_run=False)
+    def test_issue16335(self, size):
+        # very very long bogus character name
+        x = b'\\N{SPACE' + b'x' * (_testcapi.UINT_MAX + 1) + b'}'
+        self.assertEqual(len(x), len(b'\\N{SPACE}') + (_testcapi.UINT_MAX + 1))
+        self.assertRaisesRegex(UnicodeError,
+            'unknown Unicode character name',
+            x.decode, 'unicode-escape'
+        )
+
+
 def test_main():
     support.run_unittest(UnicodeNamesTest)
 
diff -r d170844a363f Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Fri Nov 09 01:09:27 2012 +0200
+++ b/Objects/unicodeobject.c	Fri Nov 09 16:36:22 2012 +0200
@@ -5561,7 +5561,8 @@
                     /* found a name.  look it up in the unicode database */
                     message = "unknown Unicode character name";
                     s++;
-                    if (ucnhash_CAPI->getcode(NULL, start, (int)(s-start-1),
+                    if (s - start - 1 <= INT_MAX &&
+                        ucnhash_CAPI->getcode(NULL, start, (int)(s-start-1),
                                               &chr, 0))
                         goto store;
                 }