title: ReactJs - Uncaught TypeError: Cannot read property 'string' of undefined
body: <p>My 2 files:</p>
  
  <p><strong>APP:</strong></p>
  
  <pre><code>import React, { Component } from 'react';
  import { NICE, SUPER_NICE } from './colors';
  import Counter from './counter';
  
  export class App extends Component {
    render() {
      return (
        &lt;div&gt;
          &lt;Counter increment={ 1 } color={ NICE } /&gt;
          &lt;Counter increment={ 5 } color={ SUPER_NICE } /&gt;
        &lt;/div&gt;
      );
    }
  }
  </code></pre>
  
  <hr>
  
  <p><strong>Counter:</strong></p>
  
  <pre><code>import React, { Component } from 'react';
  
  class Counter extends Component {
    constructor(props) {
      super(props);
      this.state = { counter: 0 };
      this.interval = setInterval(() =&gt; this.tick(), 1000);
    }
  
    tick() {
      this.setState({
        counter: this.state.counter + this.props.increment
      });
    }
  
    componentWillUnmount() {
      clearInterval(this.interval);
    }
  
    render() {
      return (
        &lt;h1 style={ { color: this.props.color } }&gt;
          Counter ( { this.props.increment } ): { this.state.counter }
        &lt;/h1&gt;
      );
    }
  }
  
  Counter.defaultProps = {
    increment: 0,
    color: null
  };
  
  Counter.propTypes = {
    increment: React.PropTypes.number.isRequired,
    color: React.propTypes.string.isRequired
  };
  
  
  module.exports = Counter;
  </code></pre>
  
  <hr>
  
  <p><strong>I am using:</strong></p>
  
  <pre><code>"react": "^0.14.0-rc1",
  "eslint": "^1.3.1",
  "eslint-config-airbnb": "^0.1.0",
  "eslint-loader": "^1.0.0",
  "eslint-plugin-react": "^3.0.0"
  </code></pre>
  
  <p>I need the declare the propTypes to satisfy eslint, but I am receiving a run-error:</p>
  
  <p><strong>"Uncaught TypeError: Cannot read property 'string' of undefined".</strong></p>
  
  <p>Does anyone see my error?</p>
  
  <p>Many Thanks !</p>