diff -r 7a90fddb13b0 -r c4dbbaecaad3 mercurial/rewriteutil.py
--- a/mercurial/rewriteutil.py	Thu Feb 11 15:11:10 2021 -0800
+++ b/mercurial/rewriteutil.py	Wed Apr 28 08:48:10 2021 -0700
@@ -44,7 +44,7 @@
         revs = (r.rev() for r in revs)
 
     if len(repo[None].parents()) > 1:
-        raise error.StateError(_(b"cannot %s while merging") % action)
+        raise error.StateError(_(b"cannot %s changesets while merging") % action)
 
     publicrevs = repo.revs(b'%ld and public()', revs)
     if publicrevs: