Thumb preloader

I have a movie clip full of thumbs there location is _root.thumbs.thumb1.thumb2,thumb3 etc. I want to make a preloader for them I have tried this code:

onEnterFrame = function(){
for (var j = 0; j<30; j++){
var pctLoaded:Number = Math.round(_root.thumbs[“thumb”+j].getBytesLoaded()/_root.thumbs[“thumb”+j].getBytesTotal()*100);

I am completely stumped as it just comes back NaN or the individual classes come back undefined.
any help would be fantastic.