diff --git a/spacy/tests/serialize/test_serialize_kb.py b/spacy/tests/serialize/test_serialize_kb.py
index 1597799d5..f9d2e226b 100644
--- a/spacy/tests/serialize/test_serialize_kb.py
+++ b/spacy/tests/serialize/test_serialize_kb.py
@@ -1,7 +1,8 @@
-import pickle
 from pathlib import Path
 from typing import Callable, Iterable, Any, Dict
 
+import srsly
+
 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
@@ -122,18 +123,13 @@ def test_serialize_subclassed_kb():
             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)
+            def serialize_custom_fields(file_path: Path) -> None:
+                srsly.write_json(file_path, {"custom_field": self.custom_field})
 
             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
-                ),
+                "custom_fields": lambda p: serialize_custom_fields(p),
             }
             util.to_disk(path, serialize, exclude)
 
@@ -146,8 +142,7 @@ def test_serialize_subclassed_kb():
                 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"]
+                self.custom_field = srsly.read_json(file_path)["custom_field"]
 
             deserialize: Dict[str, Callable[[Any], Any]] = {
                 "contents": lambda p: self.read_contents(p),