Filtering by Custom Field

Hi.

I get correct results when filtering the API on a custom field like this:

company.json?%24filter=customfield_prop_id%20eq%20123

If I search for something that shouldnt result in a match it always returns the first company/client in the list of clients. eg Changed search value to unused value:

company.json?%24filter=customfield_prop_id%20eq%2012333333