Filter the tasks which have not been accepted by any user

Have a task report for fetching the tasks assigned to a pool of users including logged in user.

We have a requirement to filter the tasks which have not been accepted by any user.

Am using following query filter in queryProcessAnalytics()

a!queryFilter(field:"c6",operator:"is null")
where "c6" denotes the Owner of the task i.e tp!owner

But its not working as expected. The report does not return anything.

Please suggest.

OriginalPostID-217325

OriginalPostID-217325