diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
index ced21a6d72af1..de8b00018e8d5 100644
--- a/src/lib/es5.d.ts
+++ b/src/lib/es5.d.ts
@@ -1205,13 +1205,13 @@ interface ReadonlyArray<T> {
      */
     slice(start?: number, end?: number): T[];
     /**
-     * Returns the index of the first occurrence of a value in an array.
+     * Returns the index of the first occurrence of a value in an array, or -1 if it is not present.
      * @param searchElement The value to locate in the array.
      * @param fromIndex The array index at which to begin the search. If fromIndex is omitted, the search starts at index 0.
      */
     indexOf(searchElement: T, fromIndex?: number): number;
     /**
-     * Returns the index of the last occurrence of a specified value in an array.
+     * Returns the index of the last occurrence of a specified value in an array, or -1 if it is not present.
      * @param searchElement The value to locate in the array.
      * @param fromIndex The array index at which to begin the search. If fromIndex is omitted, the search starts at the last index in the array.
      */