React Native How to use SectionList, but have warning: "A section you supplied is missing the key property"

MySectionList.js

/**
 * Created by Owen Ju on 22/05/2017.
 */

import React, {Component} from 'react'
import {AppRegistry, View, Text, StyleSheet, SectionList} from 'react-native'

// build mock data
const sections = [
  {
    id: 5,
    title: "Basic Components",
    data: [
      {id: 0, text: "View"},
      {id: 1, text: "Text"},
      {id: 2, text: "Image"},
    ]
  },
  {
    id: 6,
    title: "List Components",
    data: [
      {id: 3, text: "ScrollView"},
      {id: 4, text: "ListView"},
    ]
  },
]

const extractKey = (item, idx) => {
  return item.id
}

export default class MySectionList extends Component {

  renderItem = ({item}) => {
    return (
      <Text style={styles.row}>
        {item.text}
      </Text>
    )
  }

  renderSectionHeader = ({section}) => {
    return (
      <Text style={styles.header}>
        {section.title}
      </Text>
    )
  }

  render() {
    return (
      <SectionList
        style={styles.container}
        sections={sections}
        renderItem={this.renderItem}
        renderSectionHeader={this.renderSectionHeader}
        keyExtractor={extractKey}/>
    )
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
  },
  row: {
    backgroundColor: "red",
    height: 50,
  },
  header: {
    backgroundColor: 'blue',
    height: 50
  }
})


AppRegistry.registerComponent("MySectionList", () => MySectionList)

Give me a warn : A section you supplied is missing the key property
don't know