diff --git a/mercurial/dagop.py b/mercurial/dagop.py
--- a/mercurial/dagop.py
+++ b/mercurial/dagop.py
@@ -9,7 +9,6 @@ from __future__ import absolute_import
 
 import heapq
 
-from .node import nullrev
 from .thirdparty import attr
 from .node import nullrev
 from . import (