diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -36,7 +36,9 @@ def _toolbool(ui, tool, part, default=Fa
     return ui.configbool("merge-tools", tool + "." + part, default)
 
 def _toollist(ui, tool, part, default=None):
-    return ui.configlist("merge-tools", tool + "." + part, default or [])
+    if default is None:
+        default = []
+    return ui.configlist("merge-tools", tool + "." + part, default)
 
 internals = {}
 # Merge tools to document.