Uncaught TypeError: Cannot read property 'isRequired' of undefined

function Header(props){
  return(
    <div className="header">
      <h1>{props.title}</h1>
    </div>
  );
}

Header.propTypes = {
  title: React.PropTypes.strint.isRequired,
}

function Player(props){
  return(
      <div className="player">
        <div className="player-name">
          {props.name}
        </div>
        <div className="player-score">
          <div className="counter">
              <button className="counter-action decrement"> - </button>
              <div className="counter-score">{props.score}</div>
              <button className="counter-action increment"> + </button>
          </div>
        </div>
      </div>
  );
}

Player.propTypes = {
  name: React.PropTypes.string.isRequired,
  score: React.PropTypes.number.isRequired,
}

function Application(props) {
  return (
   <div className="scoreboard">

    <Header title={props.title} /> 

    <div className="players">
        <Player name="Jim Hoskins" score={31} />
        <Player name="Andrew Chalkers" score={33} />
    </div>
   </div>
  );
}

Application.propTypes = {
  title: React.PropTypes.string,
};

Application.defaultProps = {
  title: "Scoreboard",
};

ReactDOM.render(
  <Application />, document.getElementById('container')
);