--- a/mercurial/debugcommands.py	Tue Sep 19 22:01:31 2017 +0200
+++ b/mercurial/debugcommands.py	Tue Sep 19 22:23:41 2017 +0200
@@ -310,7 +310,7 @@
 def _debugphaseheads(ui, data, indent=0):
     """display version and markers contained in 'data'"""
     indent_string = ' ' * indent
-    headsbyphase = bundle2._readphaseheads(data)
+    headsbyphase = phases.binarydecode(data)
     for phase in phases.allphases:
         for head in headsbyphase[phase]:
             ui.write(indent_string)