diff --git a/mercurial/debugcommands.py b/mercurial/debugcommands.py
--- a/mercurial/debugcommands.py
+++ b/mercurial/debugcommands.py
@@ -2008,7 +2008,7 @@
             b'"}'
         )
 
-    ms = mergestatemod.mergestate.read(repo)
+    ms = repo[None].mergestate()
 
     fm = ui.formatter(b'debugmergestate', opts)
     fm.startitem()