diff --git a/less/less.d.ts b/less/less.d.ts
index 1b063e39c3..d5787913b9 100644
--- a/less/less.d.ts
+++ b/less/less.d.ts
@@ -504,7 +504,7 @@ declare module "less" {
             value: string;
 
             toCSS(env?: Options): string;
-            genCSS(env?: Options, output): string;
+            genCSS(env: Options, output): string;
             eval(): Attribute;
         }