diff --git a/spacy/tests/serialize/test_serialize_kb.py b/spacy/tests/serialize/test_serialize_kb.py
index 90fc3136f..102abb4dc 100644
--- a/spacy/tests/serialize/test_serialize_kb.py
+++ b/spacy/tests/serialize/test_serialize_kb.py
@@ -1,7 +1,9 @@
-from typing import Callable
+import pickle
+from pathlib import Path
+from typing import Callable, Iterable, Any, Dict
 
-from spacy import util
-from spacy.util import ensure_path, registry, load_model_from_config
+from spacy import util, Errors
+from spacy.util import ensure_path, registry, load_model_from_config, SimpleFrozenList
 from spacy.kb.kb_in_memory import InMemoryLookupKB
 from spacy.vocab import Vocab
 from thinc.api import Config
@@ -112,6 +114,43 @@ def test_serialize_subclassed_kb():
             super().__init__(vocab, entity_vector_length)
             self.custom_field = custom_field
 
+        def to_disk(self, path, exclude: Iterable[str] = SimpleFrozenList()):
+            """We overwrite InMemoryLookupKB.to_disk() to ensure that self.custom_field is stored as well."""
+            path = ensure_path(path)
+            if not path.exists():
+                path.mkdir(parents=True)
+            if not path.is_dir():
+                raise ValueError(Errors.E928.format(loc=path))
+
+            def serialize_custom_fields(values: Dict[str, Any], file_path: Path) -> None:
+                with open(file_path, "wb") as file:
+                    pickle.dump(values, file)
+
+            serialize = {
+                "contents": lambda p: self.write_contents(p),
+                "strings.json": lambda p: self.vocab.strings.to_disk(p),
+                "custom_fields": lambda p: serialize_custom_fields({"custom_field": self.custom_field}, p)
+            }
+            util.to_disk(path, serialize, exclude)
+
+        def from_disk(self, path, exclude: Iterable[str] = SimpleFrozenList()):
+            path = ensure_path(path)
+            if not path.exists():
+                raise ValueError(Errors.E929.format(loc=path))
+            if not path.is_dir():
+                raise ValueError(Errors.E928.format(loc=path))
+
+            def deserialize_custom_fields(file_path: Path) -> None:
+                with open(file_path, "rb") as file:
+                    self.custom_field = pickle.load(file)["custom_field"]
+
+            deserialize: Dict[str, Callable[[Any], Any]] = {
+                "contents": lambda p: self.read_contents(p),
+                "strings.json": lambda p: self.vocab.strings.from_disk(p),
+                "custom_fields": lambda p: deserialize_custom_fields(p)
+            }
+            util.from_disk(path, deserialize, exclude)
+
     @registry.misc("kb_test.CustomEmptyKB.v1")
     def empty_custom_kb() -> Callable[[Vocab, int], SubInMemoryLookupKB]:
         def empty_kb_factory(vocab: Vocab, entity_vector_length: int):
@@ -126,7 +165,7 @@ def test_serialize_subclassed_kb():
     @registry.misc("kb_test.CustomKB.v1")
     def custom_kb(
         entity_vector_length: int, custom_field: int
-    ) -> Callable[[Vocab], InMemoryLookupKB]:
+    ) -> Callable[[Vocab], SubInMemoryLookupKB]:
         def custom_kb_factory(vocab):
             kb = SubInMemoryLookupKB(
                 vocab=vocab,