--- generator.py	2016-03-31 04:49:03.000000000 -0700
+++ /home/mark/generator.py	2016-06-14 15:49:27.668328440 -0700
@@ -186,8 +186,11 @@
         # If we munged the cte, copy the message again and re-fix the CTE.
         if munge_cte:
             msg = deepcopy(msg)
-            msg.replace_header('content-transfer-encoding', munge_cte[0])
-            msg.replace_header('content-type', munge_cte[1])
+            del msg['content-transfer-encoding']
+            del msg['content-type']
+            msg['Content-Transfer-Encoding'] = munge_cte[0]
+            msg['Content-Type'] = munge_cte[1]
+                
         # Write the headers.  First we see if the message object wants to
         # handle that itself.  If not, we'll do it generically.
         meth = getattr(msg, '_write_headers', None)