diff --git a/CHANGES.rst b/CHANGES.rst
index 389ae9e7..4ed9497f 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -1,8 +1,13 @@
 Changelog
 =========
 
+Unreleased
+----------
+
+- fix type signature in ``flask_caching.utils.make_template_fragment_key``. :pr:`430`
 - Added docs and example for make_cache_key
 
+
 Version 2.0.2
 -------------
 
@@ -12,6 +17,7 @@ Released 2023-01-12
 - migrate ``flask_caching.backends.RedisCluster`` dependency from redis-py-cluster to redis-py
 - bug fix: make the ``make_cache_key`` attributed of decorated view functions writeable. :pr:`431`, issue `#97`
 
+
 Version 2.0.1
 -------------
 
diff --git a/setup.cfg b/setup.cfg
index afe6b815..65fd7aa3 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -91,7 +91,7 @@ disallow_subclassing_any = true
 # disallow_incomplete_defs = true
 # check_untyped_defs = true
 disallow_untyped_decorators = true
-# no_implicit_optional = true
+no_implicit_optional = true
 warn_redundant_casts = true
 warn_unused_ignores = true
 # warn_return_any = true
diff --git a/src/flask_caching/utils.py b/src/flask_caching/utils.py
index d499098c..b853a8b6 100644
--- a/src/flask_caching/utils.py
+++ b/src/flask_caching/utils.py
@@ -2,6 +2,7 @@
 import string
 from typing import Callable
 from typing import List
+from typing import Optional
 
 TEMPLATE_FRAGMENT_KEY_TEMPLATE = "_template_fragment_cache_%s%s"
 # Used to remove control characters and whitespace from cache keys.
@@ -102,7 +103,9 @@ def function_namespace(f, args=None):
     return ns, ins
 
 
-def make_template_fragment_key(fragment_name: str, vary_on: List[str] = None) -> str:
+def make_template_fragment_key(
+    fragment_name: str, vary_on: Optional[List[str]] = None
+) -> str:
     """Make a cache key for a specific fragment name."""
     if vary_on:
         fragment_name = "%s_" % fragment_name