diff --git a/types/mime/index.d.ts b/types/mime/index.d.ts
index 71b4677ac6..7debcca6f3 100644
--- a/types/mime/index.d.ts
+++ b/types/mime/index.d.ts
@@ -7,7 +7,7 @@
 
 export as namespace mime;
 
-export function lookup(path: string, fallback: string): string;
+export function lookup(path: string, fallback?: string): string;
 export function extension(mime: string): string;
 export function load(filepath: string): void;
 export function define(mimes: { [key: string]: any }): void;