I have a custom field in invoice..i need to get invoices when the custom field is empty..

                        i can able to get all invoice or fillter by custom field  value..

                        But I need to filter custom field by null or empty value pls help...

https://crm.zoho.com/crm/private/json/Invoices/getSearchRecords?authtoken=" + "Authtoken" + "&scope=crmapi&selectColumns=All&Customfield=(Test|=|\"\")");

this is i used..