accept Method() as Any where Method() as Null is expected

[flocsy]

ERROR> Method.mc:12:22: Argument 1 to $.Toybox.Timer.Timer.start expected to be Method() as Null but got Method() as Boolean

import Toybox.Lang;
import Toybox.Timer;

class Foo {
    function initialize() {
        foo();
    }

    var timer as Timer.Timer?;
    function foo() as Void {
        timer = new Timer.Timer();
        timer.start( method(:onNextPage), 1000, true );
    }

    function onNextPage() as Boolean {
        return true;
    }
}