diff --git a/types/index.d.ts b/types/index.d.ts
index 01c307e..b9d183e 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -30,8 +30,8 @@ declare namespace fastifyBasicAuth {
     ): void | Promise<void | Error>;
     authenticate?: boolean | { realm: string | ((req: FastifyRequest) => string) };
     header?: string;
-    strictCredentials?: boolean;
-    utf8?: boolean;
+    strictCredentials?: boolean | undefined;
+    utf8?: boolean | undefined;
   }
 
   export const fastifyBasicAuth: FastifyBasicAuth
diff --git a/types/index.test-d.ts b/types/index.test-d.ts
index af291fe..a04440c 100644
--- a/types/index.test-d.ts
+++ b/types/index.test-d.ts
@@ -79,6 +79,12 @@ app.register(fastifyBasicAuth, {
   utf8: true
 })
 
+app.register(fastifyBasicAuth, {
+  validate: () => {},
+  strictCredentials: undefined,
+  utf8: undefined
+})
+
 expectAssignable<onRequestHookHandler>(app.basicAuth)
 expectAssignable<preValidationHookHandler>(app.basicAuth)
 expectAssignable<preHandlerHookHandler>(app.basicAuth)