diff --git a/hgext/uncommit.py b/hgext/uncommit.py
--- a/hgext/uncommit.py
+++ b/hgext/uncommit.py
@@ -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))
diff --git a/tests/test-unamend.t b/tests/test-unamend.t
--- a/tests/test-unamend.t
+++ b/tests/test-unamend.t
@@ -443,8 +443,5 @@
   A left
   $ hg amend
   $ hg unamend
-  $ hg st --rev 1 --rev .
-  A right
-  R left (known-bad-output !)
-  $ hg st --rev 2 --rev .
-  A left (missing-correct-output !)
+  abort: cannot unamend merge changeset
+  [10]