hgext/uncommit.py

--- a/hgext/uncommit.py	Mon Jan 24 21:12:19 2022 -0800
+++ b/hgext/uncommit.py	Mon Jan 24 21:17:04 2022 -0800
@@ -273,6 +273,8 @@
         curctx = repo[b'.']
 
         rewriteutil.precheck(repo, [curctx.rev()], b'unamend')
+        if len(curctx.parents()) > 1:
+            raise error.InputError(_(b"cannot unamend merge changeset"))
 
         # identify the commit to which to unamend
         markers = list(predecessormarkers(curctx))