Likely it does work, but throws an error nonetheless, it seems like it is a new “feature” of 7.15, check starting from here:
http://forum.mikrotik.com/t/v7-15-3-stable-is-released/176334/1
and keep an eye on that thread, it is likely that a solution or a workaround will come out, no idea what the reason could be in your simple script, though.