title: Unhandled Rejection Type Error from API fetch
body: <p>I got a weird error while working on my Spotify Web Application. I try to save a playlist to my account, which has to fetch the <code>id</code> element from the <a href="https://api.spotify.com/v1/me" rel="noreferrer">https://api.spotify.com/v1/me</a> endpoint, then apply the playlist to your account. Everything seems fine otherwise, besided the fetch to that endpoint, which throws the error:</p>
  
  <p><code>Spotify.js:81 Uncaught (in promise) TypeError: Failed to execute 'fetch' on 'Window': The provided value is not of type '(sequence&lt;sequence&lt;ByteString&gt;&gt; or record&lt;ByteString, ByteString&gt;)'</code></p>
  
  <p>I've never seen this error before, and am not sure why it's happening. The <code>findUserId</code> method is:</p>
  
  <pre><code>findUserId() {
    if(accessToken === undefined) {
      this.getAccessToken();
    }
    console.log(accessToken);
    let userId;
    fetch(`https://api.spotify.com/v1/me`, {headers: `Bearer ${accessToken}`}
      ).then(response =&gt; {return response.json()}
      ).then(jsonResponse =&gt; {
          userId = jsonResponse.id;
      });
    console.log(userId);
    return userId;
  }
  </code></pre>