Does the Gmail API support JWT?

I want to access the Gmail API using NodeJS.

I'm using a server-to-server approach (see this) but when I execute the code below, I get a backEndError, code 500 from the Google API.

Any ideas?

var authClient = new google.auth.JWT(
    'email',
    'key.pem',
    // Contents of private_key.pem if you want to load the pem file yourself
    // (do not use the path parameter above if using this param)
    'key',
    // Scopes can be specified either as an array or as a single, space-delimited string
    ['https://www.googleapis.com/auth/gmail.readonly']
);

authClient.authorize(function(err, tokens) {
    if (err) 
        console.log(err);

    gmail.users.messages.list({ userId: 'me', auth: authClient }, function(err, resp) {
    // handle err and response
    if (err) {
        console.log(err);
});