incorrect result when using += operator in closure

a=[3:5]

class B {
        int v;
}

B b = new B();
b.v = 3

clos = {
        if (it!=null)
        {
                a[it.v] += 3
        }
}
clos.call(b)

println 'b.v = ' + b.v
println 'a = ' + a