diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -693,7 +693,7 @@
     ui.status(t.renderdefault(props))
 
 
-def _xmerge(repo, mynode, orig, fcd, fco, fca, toolconf, files, labels=None):
+def _xmerge(repo, mynode, orig, fcd, fco, fca, toolconf, files, labels):
     tool, toolpath, binary, symlink, scriptfn = toolconf
     uipathfn = scmutil.getuipathfn(repo)
     if fcd.isabsent() or fco.isabsent():