title: GraphCool TypeError: Converting circular structure to JSON
body: <p>I copy-pasted example from official documentation in hope that I will see some different error (invalid URL or invalid auth key or something similar) However I get some webpack/sandbox error:</p>
  
  <pre><code>const fetch = require('isomorphic-fetch')
  const Base64 = require('Base64')
  const FormData =require('form-data')
  
  const apiKey = '__MAILGUN_API_KEY__'
  const url = '__MAILGUN_URL__'
  
  export default event =&gt; {
  
    const form = new FormData()
    form.append('from', 'Nilan &lt;nilan@graph.cool&gt;')
    form.append('to', 'Nikolas &lt;nikolas@graph.cool&gt;')
    form.append('subject', 'Test')
    form.append('text', 'Hi')
  
    return fetch(url, {
      headers: {
        'Authorization': `Basic ${Base64.btoa(apiKey)}`
      },
      method: 'POST',
      body: form
    })
  }
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/E7Q0J.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/E7Q0J.png" alt="enter image description here"></a></p>
  
  <p>Even simple API requests fail:</p>
  
  <pre><code>require('isomorphic-fetch')
  
  module.exports = function (event) {
    const url = 'https://jsonplaceholder.typicode.com/posts'
    return fetch(url)
  }
  </code></pre>
  
  <p>The code above also returns:</p>
  
  <pre><code>TypeError: Converting circular structure to JSON
  
  at Object.stringify (native)
  
  at /data/sandbox/lib/sandbox.js:532:48
  
  at /data/io/8e0059b3-daeb-4989-972f-e0d88e27d15e/webtask.js:46:33
  
  at process._tickDomainCallback (node.js:481:9)
  </code></pre>
  
  <p>How do I successfully call API from custom graphcool subscription/resolver?</p>