diff --git a/src/awkward/contents/indexedoptionarray.py b/src/awkward/contents/indexedoptionarray.py
index c31bb43d98..274f02719b 100644
--- a/src/awkward/contents/indexedoptionarray.py
+++ b/src/awkward/contents/indexedoptionarray.py
@@ -269,7 +269,9 @@ def to_IndexedOptionArray64(self) -> IndexedOptionArray:
             return self
         else:
             return IndexedOptionArray(
-                self._backend.nplike.astype(self._index, dtype=np.int64),
+                ak.index.Index64(
+                    self._backend.nplike.astype(self._index.data, dtype=np.int64)
+                ),
                 self._content,
                 parameters=self._parameters,
             )