PFQuery find object with null value

Question

I would like to fetch all objects where paidDate is not set yet.

let query = PFQuery(className:"Salary")        
query.whereKey("paidDate", equalTo: nil)
query.findObjectsInBackground { (objects, error) in
...

I don't want to put a specific date to a query. The fact that paidDate was not set means that salary was not paid, so I would like to get all unpaid salaries.

Is this possible to make such query?