Can't get a return value from app.Invoke() in Xamarin.iOS

Folks,
I have a backdoor in my iOS app which looks like this:

[Export("myBackdoor:")]
public NSString MyBackdoor(NSString value){
    return new NSSString("Backdoor called");
}

In the app.Repl() for Xamarin.UiTest, when I say

 >>>  app.Invoke("myBackdoor:", "my param")
[

]

In Android, I have

[Export("MyBackdoor")]
public string MyBackdoor(){
    return "Backdoor called";
}

And in Repl:

>>> app.Invoke("myBackdoor")
"Backdoor called"

i.e. Android returns the value and iOS doesn't.

anyone can help, please?