Searching for null fields

Hi

I am attempting to do a search for a NULL value in a field, and every format I have tried as resulted in an error. The formats, I have used are

https://domain.freshdesk.com/api/v2/contacts?mobile:null
https://domain.freshdesk.com/api/v2/contacts?mobile=null
https://domain.freshdesk.com/api/v2/contacts?query="mobile:null"
https://domain.freshdesk.com/api/v2/contacts?query=%22mobile%3Anull%22

But I get an error to say Unexpected/invalid field in request

I have also tried

https://domain.freshdesk.com/api/v2/contacts?mobile=null
https://domain.freshdesk.com/api/v2/contacts?mobile="Null"

but these two forms return an empty set even though there are contacts where the mobile number is shown as null

Can anyone advise on this query as I am stuck

He Dave,
do not have any answer, but I had the same request aswell.

I then resolved it with a workaround, without having the need to export only those with null - But still would be interested, if there’s an option to do so.

Best
Tom