diff --git a/index.d.ts b/index.d.ts
index 287a22e..203b96e 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -5,7 +5,7 @@
 interface Options {
   debug?: boolean;
   message?: string;
-  format?: string; // MIME type
+  format?: 'text/html' | 'text/plain'; // MIME type
   onCopy?: (clipboardData: object) => void;
 }