What is wrong in the Swift block syntax below?

I am integrating AWS S3 service in my Swift based project but I stuck in the sysntax below as there is no description for the usage of API with Swift so I was trying to convert the code by myself.

transferManager.download(downloadRequest).continueWithExecutor(BFExecutor.mainThreadExecutor(), withBlock: 
{ (task:BFTask!) -> AnyObject! in
            println("test")
        })

I am getting following error!

Type '()' does not conform to protocol 'AnyObject'

Please help ASAP! Thanks!