TypeError: Cannot read property 'setState' of undefined

class Counter extends React.Component {
    constructor(props) {
      super(props);

      this.state = {
          count : 1
      };

      this.delta = this.delta.bind(this);
	}

    delta() {
        this.setState({
            count : this.state.count++
        });
    }

    render() {
        return (
            <div>
                <h1>{this.state.count}</h1>
                <button onClick={this.delta}>+</button>
            </div>
        );
    }
}