How to pass a nullable decimal from Mathematica to .Net via NETLink

Question

I have no problems invoking methods with decimal parameters, as follows:

//public void MyMethod(decimal d){

myWrapper@MyMethod[1.23];

However, I need to change the signature of MyMethod as follows:

//public void MyMethod(decimal? d){
// this call works
myWrapper@MyMethod[Null];

Passing a Null to the modified MyMethod works, but passing a decimal fails:

myWrapper@MyMethod[1.2];
NET::methodargs: Improper arguments supplied for method named MyMethod

What am I missing?