As the error suggests, it gets disabled because it tries to divide by zero.. Most probably because you are in negative time due to this: t = time - key(n).time;

You can protect against that in several ways but here's a quick cheeky way:

amp = .1;

freq = 2.0;

decay = 12.0;

n = 0;

if (numKeys > 0){ n = nearestKey(time).index;

if (key(n).time > time){ n--;

}} if (n == 0){ t = 0;

}else{ t = time - key(n).time;

} if (n > 0){ v = velocityAtTime(key(n).time - thisComp.frameDuration/10);

d= Math.exp(decay*t);

if ( d == 0 ) d= 0.0001;

 value + v*amp*Math.sin(freq*t*2*Math.PI)/d;

}else{value}

Cheers,

Lloyd

On Wednesday, September 25, 2013, D wrote:

...confusingly, the expression will sometimes continue to work on other layers in the same comp on both position and scale parameters whilst some get disabled. 

On 25 September 2013 10:40, D <dow.hanson@gmail.com> wrote:

Hi

The expression I'm using looks like this:

amp = .1;

freq = 2.0;

decay = 12.0;

n = 0;

if (numKeys > 0){ n = nearestKey(time).index;

if (key(n).time > time){ n--;

}} if (n == 0){ t = 0;

}else{ t = time - key(n).time;

} if (n > 0){ v = velocityAtTime(key(n).time - thisComp.frameDuration/10);

value + v*amp*Math.sin(freq*t*2*Math.PI)/Math.exp(decay*t);

}else{value}

It works most of the time then will randomly get disabled across lots of layers and will not allow me to re-enable until I've restarted AE (v5.5). This is the error:

--

After Effects warning: invalid numeric result (divide by zero?)

Expression disabled.

Error occurred at line 10.

Comp: 'XXX'

Layer 17 ('XXX')

Property: 'Scale'

--

I have no real knowledge of how expression code works. I cut and paste when I use them. The project is 4 minutes long and the expression occurs across many comps and typically as the deadline approaches the error is now cropping up often. Any help appreciated.

Thanks.