Can't register a custom class in Swift 2 for XPC service

I'm trying to send a custom class through an XPC service in Swift 2. It works with a class written in Objective C, however with a Swift class, using the exact same code for class registration, I get a decode exception. Here's the class registration bit :

        let interface = NSXPCInterface(withProtocol: MyXPCProtocol.self)
        let currentExpectedClasses = interface.classesForSelector("myXPCMethod:", argumentIndex: 0, ofReply: false) as NSSet
        let allClasses = currentExpectedClasses.setByAddingObject(CustomClass.self)
        interface.setClasses(allClasses as Set<NSObject>, forSelector: "myXPCMethod:", argumentIndex: 0, ofReply: false)

and myXPCMethod :

    func myXPCMethod(message:CustomClass) {
        NSLog("ping back recevied with message :\(message)");
    }

with CustomClass being a Swift class, satisfying the NSSecureCoding protocol.

When trying to call myXPCMethod from the remote end, I get this error :

Exception caught during decoding of received message, dropping incoming message.

Exception: Exception while decoding argument 0 (#2 of invocation):

<NSInvocation: 0x63000006c4c0>

return value: {v} void

target: {@} 0x0

selector: {:} myXPCMethod:

argument 2: {@} 0x0

Exception: decodeObjectForKey: class "ImageDownloader.CustomClass" not loaded or does not exist

I guess the problem comes from the "ImageDownloader" prefix (that's the name of the XPC service sending the class) but I have no idea on how to solve it.

Any ideas ?

Thanks