To add to craig_h's comment above (I currently don't have enough rep to add this as a comment to his answer, sorry), if your primary key is not an integer, you'll also want to tell your model what data type it is, by setting keyType at the top of the model definition.
public $keyType = 'string'
Eloquent understands any of the types defined in the castAttribute()
function, which as of Laravel 5.4 are: int, float, string, bool, object, array, collection, date and timestamp.
This will ensure that your primary key is correctly cast into the equivalent PHP data type.