diff --git a/sleekxmpp/thirdparty/suelta/mechanisms/cram_md5.py b/sleekxmpp/thirdparty/suelta/mechanisms/cram_md5.py
index ba44bef..e07bb88 100644
--- a/sleekxmpp/thirdparty/suelta/mechanisms/cram_md5.py
+++ b/sleekxmpp/thirdparty/suelta/mechanisms/cram_md5.py
@@ -33,7 +33,7 @@ class CRAM_MD5(Mechanism):
         if 'savepass' not in self.values:
             del self.values['password']
 
-    def process(self, challenge):
+    def process(self, challenge=None):
         """
         """
         if challenge is None: