TypeError: Cannot read property 'map' of undefined

When I enter the code as he says on the video I get the error message described in the title...

In GuestList.js const GuestList = (props) => <ul> {props.guests.map((guest, index) => <li className="responded"> <span>Iver</span> <label> <input type="checkbox" checked /> Confirmed </label> <button>edit</button> <button>remove</button> </li> )} </ul>

GuestList.propTypes = { guests: PropTypes.array.isRequired }

In App.js => <GuestList guests={this.state.guests} />

However, if I change the propTypes to say guest it works...

In GuestList.js const GuestList = (props) => <ul> {props.guests.map((guest, index) => <li className="responded"> <span>Iver</span> <label> <input type="checkbox" checked /> Confirmed </label> <button>edit</button> <button>remove</button> </li> )} </ul>

GuestList.propTypes = { guests: PropTypes.array.isRequired }

In App.js => <GuestList guests={this.state.guests} />

Can someone tell me why this is happening, and why the code from the video doesn't work?