Can you spot the deliberate mistake

gistfile1.rb

def attributes_from_bill_json(json)  
   accounting_category = find_accounting_category(json['category']),
                                                                                                                        
   {                                                                                                                                                                                             
     :accounting_category => accounting_category,                                                                                                                                                 
     :project_name => find_project_name(json['project']),                                                                                                                                             
     :contact => find_contact(json['contact'])                                                                                                                                       
   }
end

bill.update_attributes(attributes_from_bill_json(json)) #=> raises "undefined method 'stringify_keys!' for Array"??