diff --git a/github/Issue.py b/github/Issue.py
index 333facdc..741f8aa7 100644
--- a/github/Issue.py
+++ b/github/Issue.py
@@ -150,9 +150,9 @@ class Issue( GithubObject.GithubObject ):
     def edit( self, title = GithubObject.NotSet, body = GithubObject.NotSet, assignee = GithubObject.NotSet, state = GithubObject.NotSet, milestone = GithubObject.NotSet, labels = GithubObject.NotSet ):
         assert title is GithubObject.NotSet or isinstance( title, ( str, unicode ) ), title
         assert body is GithubObject.NotSet or isinstance( body, ( str, unicode ) ), body
-        assert assignee is GithubObject.NotSet or isinstance( assignee, NamedUser.NamedUser ), assignee
+        assert assignee is GithubObject.NotSet or assignee is None or isinstance( assignee, NamedUser.NamedUser ), assignee
         assert state is GithubObject.NotSet or isinstance( state, ( str, unicode ) ), state
-        assert milestone is GithubObject.NotSet or isinstance( milestone, Milestone.Milestone ), milestone
+        assert milestone is GithubObject.NotSet or milestone is None or isinstance( milestone, Milestone.Milestone ), milestone
         assert labels is GithubObject.NotSet or all( isinstance( element, ( str, unicode ) ) for element in labels ), labels
         post_parameters = dict()
         if title is not GithubObject.NotSet:
@@ -160,11 +160,11 @@ class Issue( GithubObject.GithubObject ):
         if body is not GithubObject.NotSet:
             post_parameters[ "body" ] = body
         if assignee is not GithubObject.NotSet:
-            post_parameters[ "assignee" ] = assignee._identity
+            post_parameters[ "assignee" ] = assignee._identity if assignee else ''
         if state is not GithubObject.NotSet:
             post_parameters[ "state" ] = state
         if milestone is not GithubObject.NotSet:
-            post_parameters[ "milestone" ] = milestone._identity
+            post_parameters[ "milestone" ] = milestone._identity if milestone else ''
         if labels is not GithubObject.NotSet:
             post_parameters[ "labels" ] = labels
         headers, data = self._requester.requestAndCheck(