TypeError：无法读取属性＆＃39; string＆＃39;未定义的

Question

import React, { Component } from 'react';
import PropType from 'prop-types'`
Counter.propTypes = {
caption: React.PropTypes.string.isRequired,
initValue: React.PropTypes.number.isRequired
};
Counter.defaultProps = {
caption: null,
initValue: 0
};

Cannot read property 'string' of undefined