diff -r 2daeab02b4b1 -r ac7aa96e4cd8 mercurial/filemerge.py
--- a/mercurial/filemerge.py	Wed Mar 15 15:11:04 2017 -0700
+++ b/mercurial/filemerge.py	Wed Mar 15 15:11:52 2017 -0700
@@ -36,7 +36,9 @@
     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.