According to the Idiorm docs:
Any method chain that ends in find_one() will return either a single instance of the ORM class representing the database row you requested, or false if no matching record was found.
You should check to see if your query has returned any rows before calling as_array().
For example:
$market = \ORM::for_table('category')
->where('alias', $market)
->find_one();
if($market != false)
{
$market = $market->as_array();
}
If this still gives an error then you may not have initialised Idiorm properly.