diff --git a/tests/helpers.py b/tests/helpers.py
index 15cf800..161b793 100644
--- a/tests/helpers.py
+++ b/tests/helpers.py
@@ -2,7 +2,7 @@ from __future__ import annotations
 
 import asyncio
 import unittest.mock
-from typing import Any, Generic, TYPE_CHECKING, TypeVar
+from typing import Any, ClassVar, Generic, TYPE_CHECKING, TypeVar
 
 from typing_extensions import ParamSpec
 
@@ -84,7 +84,7 @@ class UnpropagatingMockMixin(Generic[T_Mock]):
             obj_name = f"{mock_name}.{kwargs['name']}" if "name" in kwargs else f"{mock_name}()"
             raise AttributeError(f"Can't access {obj_name}, mock is sealed.")
 
-        # Propagate any other children as simple `unittest.mock.Mock` instances
+        # Propagate any other children as the `child_mock_type` instances
         # rather than `self.__class__` instances
         return self.child_mock_type(**kwargs)
 
@@ -97,9 +97,16 @@ class CustomMockMixin(UnpropagatingMockMixin[T_Mock], Generic[T_Mock]):
     * Allows using the ``spec_set`` attribute as class attribute
     """
 
-    spec_set = None
+    spec_set: ClassVar[object] = None
 
     def __init__(self, **kwargs: object):
+        # If `spec_set` is explicitly passed, have it take precedence over the class attribute.
+        #
+        # Although this is an edge case, and there usually shouldn't be a need for this.
+        # This is mostly for the sake of completeness, and to allow for more flexibility.
         if "spec_set" in kwargs:
-            self.spec_set = kwargs.pop("spec_set")
-        super().__init__(spec_set=self.spec_set, **kwargs)  # pyright: ignore[reportCallIssue]  # Mixin class, this __init__ is valid
+            spec_set = kwargs.pop("spec_set")
+        else:
+            spec_set = self.spec_set
+
+        super().__init__(spec_set=spec_set, **kwargs)  # pyright: ignore[reportCallIssue]  # Mixin class, this __init__ is valid