How do text layer expression selectors operate on character value?

I'm banging my head against my desk here.  Every other attribute I try to animate using expressions selectors works by multiplying the value returned by the selector by the value of the property in question.  But I can't make heads or tails of what an expression selector does to either Character Value or Character Offset properties.

Has anyone had any luck with these?

I'm just trying to transcribe the characters from one text layer into another using the expression:

MT = thisComp.layer("Master Text").text.sourceText; if(textIndex < MT.length) MT.charCodeAt(textIndex) else 32 

I set my character value to 1, and left the Amount property set to [100,100,100] thinking the value coming out of my expression would be multiplied by 1.  Instead, the results I get are nonsensical.  Is there something obvious that I'm missing?