Calling Edge function externally not working

I'm trying to call an Edge function externally but I keep getting bootstrap error TypeError: comp.nextSlide is not a function error

Here's my code

index.html

     var comp;

        AdobeEdge.bootstrapCallback(function(compId) {

           comp = AdobeEdge.getComposition(compId).getStage();

           comp.nextSlide();

        });

in my edge file

sym.nextSlide = function (){

     console.log('next slide called');

}

Can anyone please point me in the right direction? Thanks in advance.