Laravel MassAssignmentException from firstOrNew by id

I am attempting to save data using Laravel's firstOrNew method, which is producing a MassAssignmentException error.

I don't understand why the firstOrNew method should trigger a mass assignment exception since it is merely looking at the database, not attempting to save anything. From the docs:

The firstOrNew method, like firstOrCreate will attempt to locate a record in the database matching the given attributes. However, if a model is not found, a new model instance will be returned. Note that the model returned by firstOrNew has not yet been persisted to the database.

Why then, should my code be throwing a MAE:

$tour = Tour::firstOrNew(array('id' => $this->request['id']));

As I understand, all that the above code is saying is basically "look and see if there is a row in the database with that ID, if so then return the row, if not, then create a new object with the relevant properties". It isn't saving any data so what is the problem? Thanks.

Note that the error only occurs if the row doesn't exist.

via Chebli Mohamed