title: API Gateway stage variable always null
body: <p>I am trying to use stage variables, but I always get this error:</p>
  <pre><code>{
    &quot;logref&quot;: &quot;some_uid&quot;,
    &quot;message&quot;: &quot;Invalid stage variable value: null.  Please use values with alphanumeric characters and the symbols ' ', -', '.', '_', ':', '/', '?', '&amp;', '=', and ','.&quot;
  }
  </code></pre>
  <p>My goal is to call SNS from API gateway <strong>without the need from the caller to specify the <code>TopicArn</code> and the <code>Message</code> in the query string</strong>.</p>
  <p>So in the <strong>Integration Request</strong> I am mapping the query string <code>TopicArn</code> to <code>stageVariables.TopicArn</code> (I have tried <code>'$stageVariables.TopicArn'</code> as well).</p>
  <p>And then in the Stage variables section in AWS console I input the Name <code>TopicArn</code> and the Value <code>arn:aws:sns:my_region:my_account_id:test-topic    </code></p>
  <p>After I deployed my API I test it from the AWS console and I get this error:</p>
  <pre><code>{
    &quot;logref&quot;: &quot;some_uid&quot;,
    &quot;message&quot;: &quot;Invalid stage variable value: null.  Please use values with alphanumeric characters and the symbols ' ', -', '.', '_', ':', '/', '?', '&amp;', '=', and ','.&quot;
  }
  </code></pre>
  <p>What am I doing wrong, it his achievable?</p>