How to invoke a internal javascript method of web application in a casperjs script

I have a casperjs script to check a registration form. This registration form consist of some steps that user need to do to complete own profile. So in my casper script, i would like to invoke next_step() function from my javascript library move to next step. How can i do this? I tried in this way but it doesn't work:

casper.then(function() {
    this.evaluate(function() {
        next_step();
    });
});