Fade audio with expressions

Ok, I've taken an epxression for automactically fading opacity (which worked wonderfully) and tried to modify it to fade audio the same way.  Now, I get an error that says "After Effects warning: Expression Disabled. Error at line 0 in property....  expression result must be of dimension 2 not 1."

Here is the tweaked expression:

fadeTime = 30;

audio.audioLevelsMin = -50;

audio.audioLevelsMax = 0;

layerDuration = outPoint - inPoint;

singleFrame = thisComp.frameDuration;

animateIn = linear(time, inPoint, (inPoint + framesToTime(fadeTime)), audio.audioLevelsMin, audio.audioLevelsMax);

animateOut = linear(time, (outPoint - framesToTime(fadeTime+1)), (outPoint-singleFrame), audio.audioLevelsMax, audio.audioLevelsMin);

if(time < (layerDuration/2+inPoint)){

animateIn;

}else{

animateOut;

}

Now, opacity is a linear, 1 dimensional characteristic.  Why did that work and the updated expression for audio cause an error?