diff --git a/src/numpydantic/serialization.py b/src/numpydantic/serialization.py
index 07924eb..f901994 100644
--- a/src/numpydantic/serialization.py
+++ b/src/numpydantic/serialization.py
@@ -93,7 +93,7 @@ def _absolutize_paths(value: dict, skip: Iterable = tuple()) -> dict:
     return _walk_and_apply(value, _a_path, skip)
 
 
-def _walk_and_apply(value: T, f: Callable[[U], U], skip: Iterable = tuple()) -> T:
+def _walk_and_apply(value: T, f: Callable[[U, bool], U], skip: Iterable = tuple()) -> T:
     """
     Walk an object, applying a function
     """
diff --git a/tests/test_serialization.py b/tests/test_serialization.py
index 702dc1a..5d0b2d8 100644
--- a/tests/test_serialization.py
+++ b/tests/test_serialization.py
@@ -10,6 +10,8 @@ from typing import Callable
 import numpy as np
 import json
 
+from numpydantic.serialization import _walk_and_apply
+
 pytestmark = pytest.mark.serialization
 
 
@@ -93,3 +95,34 @@ def test_relative_to_path(hdf5_at_path, tmp_output_dir, model_blank):
 
     # shouldn't have absolutized subpath even if it's pathlike
     assert data["path"] == expected_dataset
+
+
+def test_walk_and_apply():
+    """
+    Walk and apply should recursively apply a function to everything in a nesty structure
+    """
+    test = {
+        "a": 1,
+        "b": 1,
+        "c": [
+            {"a": 1, "b": {"a": 1, "b": 1}, "c": [1, 1, 1]},
+            {"a": 1, "b": [1, 1, 1]},
+        ],
+    }
+
+    def _mult_2(v, skip: bool = False):
+        return v * 2
+
+    def _assert_2(v, skip: bool = False):
+        assert v == 2
+        return v
+
+    walked = _walk_and_apply(test, _mult_2)
+    _walk_and_apply(walked, _assert_2)
+
+    assert walked["a"] == 2
+    assert walked["c"][0]["a"] == 2
+    assert walked["c"][0]["b"]["a"] == 2
+    assert all([w == 2 for w in walked["c"][0]["c"]])
+    assert walked["c"][1]["a"] == 2
+    assert all([w == 2 for w in walked["c"][1]["b"]])