Hi,
is it not actually because your index is out of range or some other issues? watch out for the meaning of this action, it means you want to "SET" not "ADD", so if your array is empty, setting index 0 won't work.
make sure you add an event for each potential issue this action throws, this way you will make sure you are setting it up properly.
I tested here and it all works.
Bye,
Jean