error: cannot convert value of type '() -> ()' to closure result type 'String' using Swift + PromiseKit

I am new to promises in Swift and using PromiseKit to try and create a very simple response in a playground and attempt to use it. I have the following code:

import UIKit
import PromiseKit

func foo(_ error: Bool) -> Promise<String> {
  return Promise { fulfill, reject in
    if (!error) {
      fulfill("foo")
    } else {
      reject(Error(domain:"", code:1, userInfo:nil))
    }
  }
}

foo(true).then { response -> String in {
  print(response)
  }
}

However I get the following error:

error: MyPlayground.playground:11:40: error: cannot convert value of type '() -> ()' to closure result type 'String'
foo(true).then { response -> String in {