Use UnionValue For AppIntent Parameter

I'm currently trying to use the new @UnionValue macro. From what I understood, it allows multiple types for a parameter.

I created the following enum:

@UnionValue
enum IntentDuration {
    case int(Int)
    case duration(Measurement<UnitDuration>)
}

Then, I tried to use it in the following AppIntent:

struct MyIntent: AppIntent {
    static let title: LocalizedStringResource = "intent.title"
    static let description = IntentDescription("intent.description")
    
    static let openAppWhenRun: Bool = true
    
    @Parameter var duration: IntentDuration
    
    @Dependency
    private var appManager: AppManager
    
    @MainActor
    func perform() async throws -> some IntentResult {
        // My action
        
        return .result()
    }
}

However, I get the following error from Xcode at the @Parameter line:

'init()' is unavailable

Did I wrongly understand how this works? Is there another way to accept multiple types for a parameter?

I didn't manage to find any docs on this.