iOS Swift cannot set AWSSNSSubscribeInput.protocol parameter

I am using Swift for iOS app development in Xcode 6.1.1.

I am trying to programatically subscribe the app to a AWS SNS topic. As I know, you need to set:

let subscriptionRequest = AWSSNSSubscribeInput()
subscriptionRequest.protocol = "application"
subscriptionRequest.topicArn = kMyTopicArn
subscriptionRequest.endpoint = endPointARN

But the problem here is the IDE keeps telling me that

subscriptionRequest.protocol = "application"

is not acceptable and it won't even build.

Really have no idea how to solve this. Could anyone help here? THANKS!