--- a/rust/hg-cpython/src/revlog.rs	Thu May 20 08:15:57 2021 -0700
+++ b/rust/hg-cpython/src/revlog.rs	Fri May 21 17:37:53 2021 +0200
@@ -469,7 +469,10 @@
         .and_then(|m| m.get(py, "RevlogError"))
     {
         Err(e) => e,
-        Ok(cls) => PyErr::from_instance(py, cls),
+        Ok(cls) => PyErr::from_instance(
+            py,
+            cls.call(py, (py.None(),), None).ok().into_py_object(py),
+        ),
     }
 }