Cannot convert value of type 'Double' to expected argument type 'NSNumber'

[zhjp2016]

本人新手，输入如下代码：
extension Double {
var dollars : String {
let formatter: NumberFormatter = NumberFormatter()
var result: String?
formatter.numberStyle = NumberFormatter.Style.currency
result = formatter.stringFromNumber(self)
if result == nil {
return "FORMAT FAILURE!"
}
return result!
}
}
在第六行出现如题错误，求教各位大神，这是怎么原因？
谢谢了

[网络咖啡]

result = formatter.stringFromNumber(NSNumber(self))