diff -r 673191aa1724 Lib/test/test_json/test_encode_basestring_ascii.py
--- a/Lib/test/test_json/test_encode_basestring_ascii.py	Sun Feb 01 22:53:41 2015 -0800
+++ b/Lib/test/test_json/test_encode_basestring_ascii.py	Mon Feb 02 15:08:05 2015 +0200
@@ -42,6 +42,7 @@ class TestPyEncodeBasestringAscii(TestEn
 class TestCEncodeBasestringAscii(TestEncodeBasestringAscii, CTest):
     @bigaddrspacetest
     def test_overflow(self):
-        s = "\uffff"*((2**32)//6 + 1)
+        size = (2**32)//6 + 1
+        s = '\x00' * size
         with self.assertRaises(OverflowError):
             self.json.encoder.encode_basestring_ascii(s)