[Problems filtering contacts]

Hi,

I am having issues trying to return a list of the contacts that only have a contact type of Customer. 

I have tried the following requests but it returns a list of all the contacts not just those that are CUSTOMER type.

• GET https://api.accounting.sage.com/v3.1/contacts?$filter=contact_types_id eq 'CUSTOMER'
• GET https://api.accounting.sage.com/v3.1/contacts?$filter=contains(contact_type_id, 'CUSTOMER')

Any help would be greatly appreciated

Thanks