diff --git a/mercurial/state.py b/mercurial/state.py
--- a/mercurial/state.py
+++ b/mercurial/state.py
@@ -83,8 +83,17 @@
     def _read(self):
         """reads the state file and returns a dictionary which contain
         data in the same format as it was before storing"""
-        with self._repo.vfs(self.fname, 'rb') as fp:
-            return cbor.load(fp)
+        try:
+            with self._repo.vfs(self.fname, 'rb') as fp:
+                ret = cbor.load(fp)
+                if (not isinstance(ret, dict) or
+                    ret.get('magicheader') != 'HGStateFile'):
+                    raise cbor.CBORDecodeError
+                return ret
+        except cbor.CBORDecodeError:     # cbor raises an Exception
+            with self._repo.vfs(self.fname, 'rb') as fp:
+                oldfn = oldstatefilefns[self.fname]
+                return oldfn(fp)
 
     def delete(self):
         """drop the state file if exists"""