diff --git a/src/s2repoze/plugins/sp.py b/src/s2repoze/plugins/sp.py
index 471ca57..99ca55c 100644
--- a/src/s2repoze/plugins/sp.py
+++ b/src/s2repoze/plugins/sp.py
@@ -508,6 +508,8 @@ class SAML2Plugin(object):
             binding = BINDING_HTTP_REDIRECT
         else:
             post = self._get_post(environ)
+            if post.list is None:
+                post.list = []
             binding = BINDING_HTTP_POST
 
         try: