--- a/mercurial/debugcommands.py	Wed Apr 25 14:51:20 2018 -0700
+++ b/mercurial/debugcommands.py	Thu Apr 26 21:24:13 2018 +0900
@@ -462,6 +462,8 @@
 
 def _debugdisplaystyle(ui):
     ui.write(_('available style:\n'))
+    if not ui._styles:
+        return
     width = max(len(s) for s in ui._styles)
     for label, effects in sorted(ui._styles.items()):
         ui.write('%s' % label, label=label)