diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
index 143f9f47a18b0..55317f70b9186 100644
--- a/src/lib/es5.d.ts
+++ b/src/lib/es5.d.ts
@@ -1077,7 +1077,7 @@ interface Tuple<TLength extends number, TUnion> extends ReadonlyArray<TUnion> {
     /**
       * Gets or sets the length of the array. This is a number one higher than the highest element defined in an array.
       */
-    length: TLength;
+    readonly length: TLength;
 }
 
 interface Array<T> {