title: How to fix Validation error of type SubSelectionNotAllowed
body: <p>I'm getting the following error when calling a lambda function through AppSync GraphQL: </p>
  
  <pre><code>{
    "data": null,
    "errors": [
      {
        "path": null,
        "locations": [{ "line": 2, "column": 3, "sourceName": null }],
        "message": "Validation error of type SubSelectionNotAllowed: Sub selection not allowed on leaf type String of field sendSignatureRequest @ 'sendSignatureRequest'"
      }
    ]
  }
  </code></pre>
  
  <p>I haven't seen this error before. Where should I be looking to fix it? </p>