React Native: this.setState is not a function

I see a number of questions on here relating to this same issue, but it seems none match the issue I’m having, and are a bit more complex.

I am in the process of learning ReactJS and React Native. I’m in the midst of reading and following the code examples from “Learning React Native” book here: https://github.com/bonniee/learning-react-native

For some reason, calling this.setState in the code below when the handleTextChange function is called, causes the “this.SetState is not a function.” error. My question is why? Unlike other questions about this same issue, I don’t believe my call to this.stateState is buried in a callback function or if statement. Why is it undefined?

Here is my code:

class WeatherProject extends Component {
  constructor(props) {
    super(props);
    this.state = {
      zip: "",
      forecast: null
    };
  }

  _handleTextChange(event) {
    this.setState({zip: event.nativeEvent.text});
  }

    render() {
    return (
      <View style={styles.container}>
      <Text style={styles.welcome}>
      You input {this.state.zip}.
      </Text>
      <TextInput
      style={styles.input}
      onSubmitEditing={this._handleTextChange}/>
      </View>
    );
  }
}