diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -4866,8 +4866,7 @@
 
     if node:
         node = scmutil.revsingle(repo, node).node()
-
-    if not node:
+    else:
         if ui.configbool(b'commands', b'merge.require-rev'):
             raise error.Abort(
                 _(
@@ -4877,6 +4876,9 @@
             )
         node = repo[destutil.destmerge(repo)].node()
 
+    if node is None:
+        raise error.Abort(_(b'merging with the working copy has no effect'))
+
     if opts.get(b'preview'):
         # find nodes that are ancestors of p2 but not of p1
         p1 = repo.lookup(b'.')
diff --git a/tests/test-merge2.t b/tests/test-merge2.t
--- a/tests/test-merge2.t
+++ b/tests/test-merge2.t
@@ -50,4 +50,8 @@
   adding b
   created new head
 
+  $ hg merge 'wdir()'
+  abort: merging with the working copy has no effect
+  [255]
+
   $ cd ..