diff --git a/packages/cultcache-py/src/cultcache_py/documents.py b/packages/cultcache-py/src/cultcache_py/documents.py
index c1e86fe1..297101b2 100644
--- a/packages/cultcache-py/src/cultcache_py/documents.py
+++ b/packages/cultcache-py/src/cultcache_py/documents.py
@@ -29,6 +29,9 @@ except ModuleNotFoundError:
     _NESTING_FAULTS = (RecursionError,)
 
 
+_CONTAINERS = (list, tuple, dict)
+
+
 class CultCachePayloadDepthError(ValueError):
     """A payload nests deeper than `MAX_PAYLOAD_DEPTH`. Names the type, the depth reached and the limit; never the payload."""
 
@@ -43,15 +46,13 @@ class CultCachePayloadDepthError(ValueError):
 def _refuse_past_depth_limit(tree: Any, type_name: str, direction: str) -> None:
     """The one nesting count. Walks a payload tree iteratively (lists, tuples and dicts are the containers), refusing
     the first container past `MAX_PAYLOAD_DEPTH`."""
-    stack = [(tree, 1)]
+    stack = [(tree, 1)] if isinstance(tree, _CONTAINERS) else []
     while stack:
         node, depth = stack.pop()
-        if not isinstance(node, (list, tuple, dict)):
-            continue
         if depth > MAX_PAYLOAD_DEPTH:
             raise CultCachePayloadDepthError(type_name, direction)
         children = chain(node.keys(), node.values()) if isinstance(node, dict) else node
-        stack.extend((child, depth + 1) for child in children if isinstance(child, (list, tuple, dict)))
+        stack.extend((child, depth + 1) for child in children if isinstance(child, _CONTAINERS))
 
 
 def _default_encode(value: Any) -> Any:
diff --git a/packages/cultcache-py/tests/test_union_parity.py b/packages/cultcache-py/tests/test_union_parity.py
index f7378ccb..4abab1f1 100644
--- a/packages/cultcache-py/tests/test_union_parity.py
+++ b/packages/cultcache-py/tests/test_union_parity.py
@@ -504,6 +504,34 @@ class BoundsVectorTests(unittest.TestCase):
             document.decode_payload(b"[" * 100000)
         self.assertIn("refused on read", str(read.exception))
 
+    def test_a_codec_that_exhausts_the_interpreter_stack_is_the_depth_refusal_on_each_side(self) -> None:
+        def boom(value: object) -> object:
+            return boom(value)
+
+        document = define_document_type("tests.boom", encode=boom, decode=boom)
+        with self.assertRaises(CultCachePayloadDepthError) as write:
+            document.encode_payload("x")
+        self.assertEqual(
+            str(write.exception), "tests.boom payload nests 257 containers deep; the limit is 256 (refused on write)."
+        )
+        with self.assertRaises(CultCachePayloadDepthError) as read:
+            document.decode_payload(b'"x"')
+        self.assertEqual(
+            str(read.exception), "tests.boom payload nests 257 containers deep; the limit is 256 (refused on read)."
+        )
+        with self.assertRaises(CultCachePayloadDepthError) as stack:
+            surface_document.decode_payload(bytes([0x91]) * 2000 + bytes([0xC0]))
+        self.assertEqual(
+            str(stack.exception),
+            "tests.union_surface payload nests 257 containers deep; the limit is 256 (refused on read).",
+        )
+
+    def test_the_validator_sees_the_value_on_write_and_the_decoded_value_on_read(self) -> None:
+        seen: list[object] = []
+        document = define_document_type("tests.validated", validate=seen.append)
+        document.decode_payload(document.encode_payload({"a": 1}))
+        self.assertEqual(seen, [{"a": 1}, {"a": 1}])
+
     def test_a_cycle_is_refused_as_depth(self) -> None:
         loop: list = []
         loop.append(loop)