Making Light Disappear & Reappear Behind Objects Tutorial

Hi
I don't know if anyone can help, I'm quite new to using expressions and a way around my problem just escapes me.

I've been trying to use on of the tutorials (Making Light Disappear & Reappear Behind Objects Using Expressions) on Creative COW.
I can get it to work but only after and error message comes up.
The error message comes up, stops the pre-render/render, then i click back in the expression window (on the layer), click back out, and its ok.
What this does mean is that i can never render off a movie as it disables the expression until i click back into and out of the box again.

The tutorial is:
Making Light Disappear & Reappear Behind Objects Using Expressions

and the script within the tutorial that is causing the problems is:

target = thisComp.layer("STAGE");
x = target.sampleImage(thisComp.layer("Null 1").transform.position, [50,50], true, time);
fade = x[3] * 100;
linear (fade, 0, 36, 80, 0);

The error message i get is:

After Effects warning: Timeout while waiting for
the engine
Expression disabled.

Error occured at line 4.


Any help will be gretly appreciated, I've tried many work arounds, tried it on different lighting effects but this error happens every time.
Could it be that my Mac is just not beefy enough?

thanks