Cannot bind to an attribute from a role

[lizmat]

role A {
    has Int $.a
}
class B does A {
    method b(\a) {
       $!a := a
    }
}
B.new.b(42)  # Type check failed in binding; expected NQPMu but got Int (42)

If we replace the $!a := a with nqp::bind($!a,a), then all is good.