Laravel creating default object from empty value error

I'm getting a 'creating default object from empty value' error.

$roomtype = Roomtype::where('name', '=' ,$roomtype_name)->first();

$roomtype -> availability -=1 ;
$roomtype -> save();
return $roomtype;

$roomtype is not returning NULL.

$roomtype->availabilty-=1

also returns a value. I tried this in php artisan tinker and it works perfectly.
Then why is it not working in my controller?

via Sai Krishna