diff --git a/src/jax_loop_utils/metrics.py b/src/jax_loop_utils/metrics.py
index 64f99b4..7c63c6b 100644
--- a/src/jax_loop_utils/metrics.py
+++ b/src/jax_loop_utils/metrics.py
@@ -57,17 +57,17 @@ def evaluate(variables_p, test_ds):
 """
 
 from __future__ import annotations
-from collections.abc import Mapping, Sequence
-from typing import Any, TypeVar, Protocol
 
-from absl import logging
+from collections.abc import Mapping, Sequence
+from typing import Any, Protocol, TypeVar
 
-from jax_loop_utils.internal import utils
-import jax_loop_utils.values
-from jax_loop_utils.internal import flax
 import jax
 import jax.numpy as jnp
 import numpy as np
+from absl import logging
+
+import jax_loop_utils.values
+from jax_loop_utils.internal import flax, utils
 
 Array = jax.Array
 ArrayLike = jax.typing.ArrayLike
@@ -663,7 +663,11 @@ def unreplicate(self: C) -> C:
 
 
 # Sentinel to make LastValue.__init__ support tree manipulations that use None.
-_default = object()
+class _DefaultSentinel:
+    pass
+
+
+_default = _DefaultSentinel()
 
 
 @flax.struct.dataclass
@@ -683,11 +687,11 @@ class LastValue(Metric):
     total: jnp.ndarray
     count: jnp.ndarray
 
-    def __init__(  # pytype: disable=missing-parameter  # jnp-array
+    def __init__(
         self,
-        total: jnp.ndarray | _default = _default,
-        count: jnp.ndarray | _default = _default,
-        value: jnp.ndarray | _default = _default,
+        total: jnp.ndarray | _DefaultSentinel = _default,
+        count: jnp.ndarray | _DefaultSentinel = _default,
+        value: jnp.ndarray | _DefaultSentinel = _default,
     ):
         """Backward compatibility constructor.