diff -r 482590320549 Lib/csv.py
--- a/Lib/csv.py	Sun Aug 25 19:06:16 2013 -0700
+++ b/Lib/csv.py	Fri Aug 30 20:54:02 2013 -0700
@@ -143,7 +143,7 @@
 
     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 = rowdict.keys() - self.fieldnames
             if wrong_fields:
                 raise ValueError("dict contains fields not in fieldnames: "
                                  + ", ".join(wrong_fields))
diff -r 482590320549 Lib/test/test_csv.py
--- a/Lib/test/test_csv.py	Sun Aug 25 19:06:16 2013 -0700
+++ b/Lib/test/test_csv.py	Fri Aug 30 20:54:02 2013 -0700
@@ -579,6 +579,12 @@
         fileobj = StringIO()
         self.assertRaises(TypeError, csv.DictWriter, fileobj)
 
+    def test_write_long(self):
+        fileobj = StringIO()
+        writer = csv.DictWriter(fileobj, ['f1','f2'], extrasaction="raise")
+        dictrow = {'f0':0, 'f1':1, 'f2':2, 'f3':3}
+        self.assertRaises(ValueError, csv.DictWriter.writerow, writer, dictrow)
+
     def test_read_dict_fields(self):
         with TemporaryFile("w+") as fileobj:
             fileobj.write("1,2,abc\r\n")