diff -r 0d6137891114 -r cd10171d6c71 mercurial/cmdutil.py
--- a/mercurial/cmdutil.py	Sat Mar 19 16:46:15 2016 -0700
+++ b/mercurial/cmdutil.py	Sat Mar 19 16:37:47 2016 -0700
@@ -2227,6 +2227,8 @@
     for name, key in edgetypes.items():
         # experimental config: ui.graphstyle.*
         styles[key] = ui.config('ui', 'graphstyle.%s' % name, styles[key])
+        if not styles[key]:
+            styles[key] = None
     for rev, type, ctx, parents in dag:
         char = formatnode(repo, ctx)
         copies = None