diff --git a/mercurial/state.py b/mercurial/state.py
--- a/mercurial/state.py
+++ b/mercurial/state.py
@@ -77,8 +77,16 @@
     def _read(self):
         """reads the evolvestate 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):
+                        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 evolvestate file if exists"""