diff --git a/src/xtream.ts b/src/xtream.ts
index 4dfa253..9073380 100644
--- a/src/xtream.ts
+++ b/src/xtream.ts
@@ -49,7 +49,7 @@ type Options = {
   url: string;
   username: string;
   password: string;
-  preferredFormat?: string;
+  preferredFormat?: 'ts' | 'm3u8' | 'rtmp';
 };
 
 /**
diff --git a/tests/main.test.ts b/tests/main.test.ts
index 48cb6fe..8659448 100644
--- a/tests/main.test.ts
+++ b/tests/main.test.ts
@@ -219,6 +219,7 @@ describe('Xtream API', () => {
       url: 'http://example.com',
       username: 'test',
       password: 'password',
+      // @ts-expect-error: this should fail due to type checking
       preferredFormat: 'nono',
     });