diff -r c38e9248f531 -r 53fe5a1a92bd hgext/narrow/narrowmerge.py
--- a/hgext/narrow/narrowmerge.py	Tue Feb 13 18:00:59 2018 -0500
+++ b/hgext/narrow/narrowmerge.py	Tue Feb 13 13:37:43 2018 -0500
@@ -28,7 +28,9 @@
         nooptypes = set(['k']) # TODO: handle with nonconflicttypes
         nonconflicttypes = set('a am c cm f g r e'.split())
         narrowmatch = repo.narrowmatch()
-        for f, action in actions.items():
+        # We mutate the items in the dict during iteration, so iterate
+        # over a copy.
+        for f, action in list(actions.items()):
             if narrowmatch(f):
                 pass
             elif not branchmerge: