--- a/tests/test-rename-after-merge.t	Fri Dec 20 15:50:13 2019 -0800
+++ b/tests/test-rename-after-merge.t	Tue Jan 28 14:07:57 2020 -0800
@@ -120,4 +120,10 @@
   $ hg log -r tip -C -v | grep copies
   copies:      b2 (b1)
 
+Test unmarking copies in merge commit
+
+  $ hg copy --forget --at-rev . b2
+  abort: cannot unmark copy in merge commit
+  [255]
+
   $ cd ..