In that case, I'm not entirely sure what causes it to return nil error.
Is there code in original Blizzard code that increases that background's alpha when OOR? because otherwise the lua function only changes the alpha for the raid frame cell itself and leaves the background untouched, either way. so that would probably be the way to go if all that was wanted was to increase the effects of what Blizzard's UI already does.
(also I don't really ever bother checking xml tbh, it was just a guess on my part since the original Blizzard code doesn't touch this, while this custom code attempts to touch it and reports a nil error
)