React 'cannot read property of undefined' when using map

I'm making a very basic React app from teamtreehouse.com, and I'm constantly encountering

"TypeError: Cannot read property 'onPlayerScoreChange' of undefined"

even though I'm binding my functions correctly (I think)

'onPlayerScoreChange' Is a method in the Grandparent component which executes when a user hits a '+' or '-' button to change a player's score.

It would be really helpful if someone could explain what is wrong, because I think I am setting this.onPlayerScoreChange = this.onPlayerScoreChange.bind(this) in the great grandparent's constructor.

Parent component:

class App extends React.Component {
constructor(props) {
    super(props);
    this.onPlayerScoreChange = this.onPlayerScoreChange.bind(this)
    this.state = {
        initialPlayers: props.initialPlayers,
    };
}

onPlayerScoreChange(delta, index) {
    this.setState((prevState, props) => {
        return {initialPlayers: this.prevState.initialPlayers[index].score += delta}
    })
}

render() {
    return(
        <div className = "scoreboard">
            <Header title = {this.props.title}/>
            <div className = "players">
                {this.state.initialPlayers.map(function(player, index) {
                    return(
                        <Player 
                        name = {player.name} 
                        score = {player.score} 
                        key = {player.id} 
                        index = {index}
                        onScoreChange = {this.onPlayerScoreChange}
                        />
                    )
                })}
            </div>
        </div>
    )
}}

(Component has default props for title)

Child component:

class Player extends React.Component {
render() {
    return(
        <div className = "player">
            <div className = "player-name">
                {this.props.name}
            </div>
            <div className = "player-score">
                <Counter score = {this.props.score} onChange = {this.props.onScoreChange} index = {this.props.index}/>
            </div>
        </div>
)
}}

Grandchild component:

class Counter extends React.Component {
constructor(props) {
    super(props)
    this.handleDecrement = this.handleDecrement.bind(this)
    this.handleIncrement = this.handleIncrement.bind(this)
}

handleDecrement() {
    this.props.onChange(-1, this.props.index)
}

handleIncrement() {
    this.props.onChange(1, this.props.index)
}

render() {
    return(
        <div className = "counter">
            <button className = "counter-action decrement" onClick = {this.handleDecrement}> - </button>
            <div className = "counter-score"> {this.props.score} </div>
            <button className = "counter-action increment" onClick = {this.handleIncrement}> + </button>
        </div>
)}}

Thank you!