Need help with Nil values

[TS xx]

Dear perl 6 users,


I am trying to pass a Nil value to a method new of a class that expects a Str.

The method new will assign this Nil value to a Str member variable. Then, this 
value will change during runtime.


At the moment I am getting this error: Type check failed in binding $value; 
expected Str but got Nil


Here is an simplified example of what I'm doing:


class MyClass {
    has Str $.value is rw;

    method new (Str $value) {
        return self.bless(:$value);
    }

    submethod BUILD (Str :$!value) {
    }
}


$myObject = MyClass.new(Nil); # << this triggers the error



I expect $.value to hold Strings, but I want to be able to instantiate MyClass 
whether I have a value already or not, and I also want to be able to tell if 
$.value has a real String or not. Is this possible?


If anyone knows a way to do it or has any clue to solve this case, I would be 
glad to hear about it.


Thanks to all in advance,

Emiliano