--- a/hgext3rd/topic/__init__.py	Fri Mar 16 15:15:17 2018 +0530
+++ b/hgext3rd/topic/__init__.py	Fri Mar 16 15:21:05 2018 +0530
@@ -244,6 +244,8 @@
     except ValueError:
         if self.obsolete():
             succ = evolvebits._singlesuccessor(self._repo, self)
+            if succ not in revlist:
+                return None
             return revlist.index(succ)
     except IndexError:
         # Lets move to the last ctx of the current topic