Empty Associative Aarray Literal

[Jack Stouffer via Digitalmars-d-learn]

I want to have one of the parameters on a function be optional. 
The problem is, is that it's a AA and D does not seem to support 
empty AA literals. Observe:

string func(int a, int[int] b = []) {
    return "mem1";
}
void main() {
    func(1);
}

$ dmd test
test.d(8): Error: cannot implicitly convert expression ([]) of 
type void[] to int[int]

This doesn't work either

string func(int a, int[int] b = int[int].init) {
    return "mem1";
}

void main() {
    func(1);
}