diff --git a/letsencrypt/revoker.py b/letsencrypt/revoker.py
index 239879542..32c6f003d 100644
--- a/letsencrypt/revoker.py
+++ b/letsencrypt/revoker.py
@@ -288,12 +288,12 @@ class Revoker(object):
             :class:`letsencrypt.revoker.Cert`
 
         """
-        list_path2 = tempfile.mktemp(".tmp", "LIST")
+        newfile_handle, list_path2 = tempfile.mkstemp(".tmp", "LIST")
         idx = 0
 
         with open(self.list_path, "rb") as orgfile:
             csvreader = csv.reader(orgfile)
-            with open(list_path2, "wb") as newfile:
+            with os.fdopen(newfile_handle, "wb") as newfile:
                 csvwriter = csv.writer(newfile)
 
                 for row in csvreader: