PFUser logInWithUsernameInBackground in Swift - Missing argument for parameter 'target' call

I am implementing the login function for Parse written in Swift. I am getting an error:

Missing argument for parameter 'target' call

It doesn't seem like I"m missing any parameters though - I'm following the declaration in PFUser.h.

Here is my code:

PFUser.logInWithUsernameInBackground(username, password: userPassword) {
    (user: PFUser?, error: NSError?) -> Void in {
        //More code here
    }
}

Thank you for your help!