--- a/src/pyams_sequence/reference.py	Thu Aug 23 13:51:32 2018 +0200
+++ b/src/pyams_sequence/reference.py	Thu Aug 23 13:52:05 2018 +0200
@@ -88,6 +88,8 @@
             visible_versions = versions.get_versions(workflow.published_states, sort=True)
             if visible_versions:
                 return visible_versions[-1]
+            else:
+                return None
         publication_info = IWorkflowPublicationInfo(content, None)
         if publication_info is not None:
             return content if publication_info.is_visible() else None