title: Swift Firebase Functions ERROR: NSCocoaErrorDomain
body: <p>When I try to run a Firebase Function from Swift, I get an error saying:</p>
  
  <p><em>Error Domain=NSCocoaErrorDomain Code=3840 "JSON text did not start with array or object and option to allow fragments not set." UserInfo={NSDebugDescription=JSON text did not start with array or object and option to allow fragments not set.}</em></p>
  
  <p>This is the Firebase Function:</p>
  
  <pre><code>exports.helloWorld = functions.https.onRequest((request, response) =&gt; {
    response.send("Hello from Firebase!")
  })
  </code></pre>
  
  <p>This is my Swift code:</p>
  
  <pre><code>Functions.functions().httpsCallable("helloWorld").call { (result, error) in
    if error == nil {
      print("Success: \(result?.data)")
    } else {
      print(error.debugDescription)
    }
  }
  </code></pre>