Getting "Invalid JSON for postcard"

[rei-gun]

I have an Android app that is sending a token to my Lumen service. Here is the code in Lumen:

use Facebook\Facebook as Facebook;

private function sendUserFbToken($userToken)
{
    $fb = new Facebook([
        'client_id' => env("FACEBOOK_APP_ID"),
        'client_secret' => env("FACEBOOK_APP_SECRET"),
        'default_graph_version' => 'v2.1',
    ]);
    try {
            $response = $fb->get('/me', $userToken);
            dd($response);
            $me = $response->getGraphUser();
            return $me;
    } catch(Facebook\Exceptions\FacebookResponseException $e) {
            echo 'Graph returned an error: ' . $e->getMessage();
          exit;
        } catch(Facebook\Exceptions\FacebookSDKException $e) {
          echo 'Facebook SDK returned an error: ' . $e->getMessage();
          exit;
        }
    }

I keep getting {"error":{"message":"Invalid JSON for postcard","code":190,"status_code":500}} in the response.

I've tried json_encode($userToken) but I get the same error.

Thanks for any help you can provide!

[rei-gun]

Turns out I've been receiving app token instead of user's token. Oops