function parameters: is it possible to pass byref ... while being optional at the same time ?

[someone via Digitalmars-d-learn]

The following gives me a compiler error when I add the second 
parameter:
is not an lvalue and cannot be modified

```d
public bool add(
   ref classTickerID robjTickerID,
   ref classExchanges robjExchanges = null /// needing this 
optional
   ) {
}
```

If I take out the null then the parameter is assumed mandatory of 
course.