diff --git a/ppa/in_memory.py b/ppa/in_memory.py
index 8ea1ff1..42867fd 100644
--- a/ppa/in_memory.py
+++ b/ppa/in_memory.py
@@ -7,13 +7,11 @@ def in_memory_repository(
     entity_already_exists_exception: Type[Exception] = ValueError,
     entity_not_found_exception: Type[Exception] = ValueError,
 ):
-    if find_by_fields is None:
-        find_by_fields = []
     def decorator(cls) -> Type:
         class InMemoryRepository(cls):
             def __init__(self) -> None:
                 self._store: dict[str, Any] = {}
-                for key in find_by_fields:
+                for key in find_by_fields or []:
                     setattr(self, f"find_by_{key}", self._make_find_by_method(key))
 
             def add(self, entity: Any) -> Any:
diff --git a/ppa/test/test_in_memory.py b/ppa/test/test_in_memory.py
index d735cd5..45cebee 100644
--- a/ppa/test/test_in_memory.py
+++ b/ppa/test/test_in_memory.py
@@ -1,3 +1,5 @@
+from typing import Any
+
 import pytest
 
 from ppa.in_memory import in_memory_repository
@@ -7,14 +9,18 @@ class User:
     def __init__(self, id_: int, name: str):
         self.id = id_
         self.name = name
+
+
 @pytest.fixture
 def user():
-    return User(1, 'john')
+    return User(1, "john")
+
 
 @in_memory_repository(find_by_fields=[])
 class UserRepository:
     pass
 
+
 @pytest.fixture
 def user_repository():
     return UserRepository()
@@ -25,13 +31,14 @@ def test_in_memory_repository_find_by_fields(user: User):
     class FieldsRepository:
         pass
 
-    rep = FieldsRepository()
+    rep: Any = FieldsRepository()
 
     rep.add(user)
 
     assert rep.find_by_id(user.id) == [user]
     assert rep.find_by_name(user.name) == [user]
 
+
 def test_in_memory_repository_exceptions(user: User):
     class CustomException1(Exception):
         pass
@@ -46,8 +53,7 @@ class CustomException2(Exception):
     class ExceptionsRepository:
         pass
 
-
-    rep = ExceptionsRepository()
+    rep: Any = ExceptionsRepository()
 
     with pytest.raises(CustomException2):
         rep.delete(user.id)
@@ -57,7 +63,7 @@ class ExceptionsRepository:
         rep.add(user)
 
 
-def test_in_memory_repository_methods(user_repository: UserRepository, user: User):
+def test_in_memory_repository_methods(user_repository: Any, user: User):
     user_repository.add(user)
     assert user_repository.retrieve(user.id) == user
     updated_user = User(id_=user.id, name="john_updated")
@@ -69,17 +75,17 @@ def test_in_memory_repository_methods(user_repository: UserRepository, user: Use
         assert user_repository.retrieve(user.id)
 
 
-def test_in_memory_repository_add_already_exists(user_repository: UserRepository, user: User):
+def test_in_memory_repository_add_already_exists(user_repository: Any, user: User):
     user_repository.add(user)
     with pytest.raises(ValueError):
         user_repository.add(user)
 
-def test_in_memory_repository_update_not_found(user_repository: UserRepository, user: User):
+
+def test_in_memory_repository_update_not_found(user_repository: Any, user: User):
     with pytest.raises(ValueError):
         user_repository.update(user)
 
 
-def test_in_memory_repository_delete_not_found(user_repository: UserRepository, user: User):
+def test_in_memory_repository_delete_not_found(user_repository: Any, user: User):
     with pytest.raises(ValueError):
         user_repository.delete(user)
-