diff -r d9a3d23cf8f0 Lib/email/headerregistry.py
--- a/Lib/email/headerregistry.py	Tue Oct 14 00:53:13 2014 +0200
+++ b/Lib/email/headerregistry.py	Tue Oct 14 16:09:48 2014 +0100
@@ -7,6 +7,7 @@
 and will probably change some before that happens.
 
 """
+from types import MappingProxyType
 
 from email import utils
 from email import errors
@@ -456,7 +457,7 @@
 
     @property
     def params(self):
-        return self._params.copy()
+        return MappingProxyType(self._params)
 
 
 class ContentTypeHeader(ParameterizedMIMEHeader):