--- a/hgext3rd/evolve/utility.py	Sun May 03 15:39:56 2020 +0800
+++ b/hgext3rd/evolve/utility.py	Thu May 07 19:47:59 2020 -0700
@@ -137,7 +137,10 @@
     topological branches and user didn't choose any evolve destination,
     in which case return (False, '.')
     """
-    targets = obsutil.successorssets(repo, ctx.node())[0]
+    succs = obsutil.successorssets(repo, ctx.node())
+    if not succs:
+        return (False, '.')
+    targets = succs[0]
     assert targets
     targetrevs = [repo[r].rev() for r in targets]
     heads = repo.revs(b'heads(%ld::%ld)', targetrevs, targetrevs)