diff -r 267ad2ed4138 Lib/csv.py
--- a/Lib/csv.py	Mon Nov 11 14:50:13 2013 -0500
+++ b/Lib/csv.py	Tue Nov 12 16:57:37 2013 +0800
@@ -143,7 +143,8 @@
 
     def _dict_to_list(self, rowdict):
         if self.extrasaction == "raise":
-            wrong_fields = [k for k in rowdict if k not in self.fieldnames]
+            wrong_fields = [repr(k) for k in rowdict if k not in
+                            self.fieldnames]
             if wrong_fields:
                 raise ValueError("dict contains fields not in fieldnames: "
                                  + ", ".join(wrong_fields))
diff -r 267ad2ed4138 Lib/test/test_csv.py
--- a/Lib/test/test_csv.py	Mon Nov 11 14:50:13 2013 -0500
+++ b/Lib/test/test_csv.py	Tue Nov 12 16:57:37 2013 +0800
@@ -579,6 +579,17 @@
         fileobj = StringIO()
         self.assertRaises(TypeError, csv.DictWriter, fileobj)
 
+    def test_write_fields_not_in_fieldnames(self):
+        with TemporaryFile("w+", newline='') as fileobj:
+            writer = csv.DictWriter(fileobj, fieldnames = ["f1", "f2", "f3"])
+            with self.assertRaises(ValueError) as cx:
+                writer.writerow({"f4": 10, "f2": "spam", 1: "abc"})
+            exception = str(cx.exception)
+            self.assertIn("dict contains fields not in fieldnames: ", exception)
+            self.assertIn("'f4'", exception)
+            self.assertNotIn("'f2'", exception)
+            self.assertIn("1", exception)
+
     def test_read_dict_fields(self):
         with TemporaryFile("w+") as fileobj:
             fileobj.write("1,2,abc\r\n")