title: Property value expected type of string but got null
body: <p>I have an <code>Icon.jsx</code> component:</p>
  
  <pre><code>import React from 'react'; import { css } from 'emotion';
  
  import ArrowRight from "./arrow-right.svg";
  
  export const iconTypes = {
      arrowRight: 'ARROW_RIGHT',
      //arrowLeft: "ARROW_LEFT", }
  
  const iconSrc = {
      ARROW_RIGHT: ArrowRight,
      ARROW_LEFT: ArrowLeft, }
  
  
  export default ({ type }) =&gt; {
      return (
          &lt;Icon&gt;
              &lt;img src={iconSrc[type]} /&gt;
          &lt;/Icon&gt;
      )
  };
  </code></pre>
  
  <p>And an `Icon.jsx' story:</p>
  
  <pre><code>import React from "react";
  import { storiesOf } from "@storybook/react";
  import { action } from "@storybook/addon-actions";
  import Icon from "../components/Icon/Index";
  import { iconTypes } from "../components/Icon/Index";
  
  
  storiesOf("Icon", module)
      .add("with text", () =&gt; (
          &lt;Icon type={iconTypes.leftArrow}&gt;
              &lt;/Icon&gt;
      ));
  </code></pre>
  
  <p>I keep getting the following error on the <code>Icon.jxs</code> component:</p>
  
  <pre><code> Property value expected type of string but got null
  </code></pre>
  
  <p>But I can't figure it out, any help would be much appreciated.</p>