Control Motion Tile with a Slider Control

My project is to create a scrolling number system similar to that of an old gas pump where the numbers roll vertically. I'm using the motion tile effect to achieve this. The scroll is nested in a precomp and I'd like to control it from my main comp using the Slider Control on a null. I can get the slider to work until I set a keyframe. Once I set a keyframe the slider no longer controls the Motion Tile. 

This is the expression I'm using. 

y = effect("Motion Tile")("Tile Center")[1]

x = 75;

S=comp("Scroll Animations Main").layer("Null 2").effect("Rank Counter")("Slider");

[x, S]