diff -r 9e61563edb67 Lib/csv.py
--- a/Lib/csv.py	Sun Aug 11 16:48:44 2013 -0700
+++ b/Lib/csv.py	Wed Aug 14 21:14:30 2013 -0700
@@ -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]
+            fieldset = set(self.fieldnames)
+            wrong_fields = [k for k in rowdict if k not in fieldset]
             if wrong_fields:
                 raise ValueError("dict contains fields not in fieldnames: "
                                  + ", ".join(wrong_fields))