diff --git a/marge/merge_request.py b/marge/merge_request.py
index bf1aaf9e..e0c1b25b 100644
--- a/marge/merge_request.py
+++ b/marge/merge_request.py
@@ -177,7 +177,7 @@ def assign_to(self, user_id):
         ))
 
     def unassign(self):
-        return self.assign_to(None)
+        return self.assign_to(0)
 
     def fetch_approvals(self):
         # 'id' needed for for GitLab 9.2.2 hack (see Approvals.refetch_info())
diff --git a/tests/test_merge_request.py b/tests/test_merge_request.py
index 83314dde..786fb080 100644
--- a/tests/test_merge_request.py
+++ b/tests/test_merge_request.py
@@ -82,7 +82,7 @@ def test_assign(self):
 
     def test_unassign(self):
         self.merge_request.unassign()
-        self.api.call.assert_called_once_with(PUT('/projects/1234/merge_requests/54', {'assignee_id': None}))
+        self.api.call.assert_called_once_with(PUT('/projects/1234/merge_requests/54', {'assignee_id': 0}))
 
     def test_rebase_was_not_in_progress_no_error(self):
         expected = [