React this.setState is not a function

I'm new in React and I'm attempting to write an application working with an API. I continue to get this mistake:

at the point when I attempt to deal with the API response. I speculate it's some kind of problem with this limiting yet I can't sort out some way to fix it. Here's the code of my component:

var AppMain = React.createClass({

    getInitialState: function() {

        return{

            FirstName: " "

        };

    },

    componentDidMount:function(){

        VK.init(function(){

            console.info("API initialisation successful");

            VK.api('users.get',{fields: 'photo_50'},function(data){

                if(data.response){

                    this.setState({ //the error happens here

                        FirstName: data.response[0].first_name

                    });

                    console.info(this.state.FirstName);

                }



            });

        }, function(){

        console.info("API initialisation failed");



        }, '5.34');

    },

    render:function(){

        return (

            <div className="appMain">

            <Header  />

            </div>

        );

    }

});