Using Mockito doAnswer in Kotlin

what would be the Kotlin equivalent to this Java code?

doAnswer(new Answer() {
    @Override
    public Object answer(InvocationOnMock invocation) throws Throwable {
        Design design = new Design();
        GetDesign.Listener callback = (GetDesign.Listener) invocation.getArguments()[0];
        callback.onSuccess(design);
        return null;
    }
}).when(someRepository).getDesign(any(GetDesign.Listener.class));

I've tried several options with lambdas but can't find an equivalent that works. For the moment I'm trying this one (it does not work)

doAnswer { invocation ->
        val design = Design()
        (invocation.arguments.elementAt(0) as GetDesign.Listener).onSuccess(design)
        null
}.`when`(someRepository).execute(listener)

Thanks in advance