values & bind problem

[Mikael Pesonen]

Hi, why is this bind not working? This returns an empty column for ?ga. 
What is the correct way to add ?gb?

SELECT * WHERE
{
        VALUES ?g { <https://example.com/a> }
        GRAPH ?g
        {
                 ?s a :blah .
                BIND(CONCAT(STR(?g), "/b") AS ?gb )
        }
}