How do you reference variables in an AA of Variants?

[Enjoys Math via Digitalmars-d-learn]

This:   
        double b = 1.0;

        Variant[string] aa = ["b": &b];
        writeln(aa["b"]);

fails with:

Error: cannot implicitly convert expression(["b":&b]) of type 
double*[string] to VariantN!20u[string]

Helps please!