Searching for null fields


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"mobile:null"

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

I have also tried"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.