Platform-independent way to check for array overrun

In my multiplatform application I have an inline class that looks something like this:

inline class Foo(val content: IntArray) {
    operator fun get(i: Int) = content[i]

In my unit tests I have a test that ensures that an overrun will fail:

fun testIncorrectIndex() {
    val foo = ... // Get an instance of Foo here. The array is 10 elements long.
    assertFails {

I have just implemented support for Javascript, and now I have a problem. In JS, the array dereferencing does not throw an exception when the index is outside the allowable values.

What I did was to add an extra check, like so:

assertFails {
    foo[10] as Int

The explicit type cast will trigger an error if the dereferencing returned undefined. However, the compiler warns me that this is a useless cast and will be very insistent that I remove it.

Is there a proper way to handle this case?