diff --git a/README.md b/README.md
index 4f0b31d..ce038b0 100644
--- a/README.md
+++ b/README.md
@@ -121,7 +121,7 @@ take effect globally.
 ```python
 def my_key_builder(
         func,
-        namespace: Optional[str] = "",
+        namespace: str = "",
         request: Request = None,
         response: Response = None,
         *args,
diff --git a/fastapi_cache/key_builder.py b/fastapi_cache/key_builder.py
index 852e632..a43317c 100644
--- a/fastapi_cache/key_builder.py
+++ b/fastapi_cache/key_builder.py
@@ -7,7 +7,7 @@ from starlette.responses import Response
 
 def default_key_builder(
     func: Callable,
-    namespace: Optional[str] = "",
+    namespace: str = "",
     request: Optional[Request] = None,
     response: Optional[Response] = None,
     args: Optional[tuple] = None,