how to call CLR ctor with default arg value null


rClr is great and thanks! Is there a way to instantiate an obj where the ctor has an arg with default value null?
in C#, I could do
obj = new TypeName( "A", "B", null )
where the ctor takes 3 string args with the last having default null.
I did not see the actual source code or header for the ctor. But sample C# like above would work.
How'd I pass from R something that ends to be null in CLR?
By the way, properties in this class are all immutable and validated on ctor. So the only way I could is to pass in ctor what I want. Having the last arg null is valid indication for wildcard.