Question from the React (legacy) test

Here is a component supposed to display a counter which increments when the button is clicked.

class Counter extends React.Component{
    constructor(){
        this.state={counter: 0};
    }
    
    increment(){
        this.setState({
            counter: this.state.counter++
        });
    }

    render(){
        return(
           <div>
               <p>{this.state.counter}</p>
               <button onClick={this.increment}>INCREMENT</button>
           </div>
        );
    }
}

What is(are) the problem(s)?