Working with tuples in Halide

I want to extract one channel in Halide,

Halide::Image<uint8_t> input = load_image("images/rgb.png");
Halide::Var x, y;
Halide::Func green;
green(x,y)= {0, input(x, y, 1), 0};

Halide::Image<uint8_t> output =
        green.realize(input.width(), input.height());

gives

Can only cast single-element realizations to buffers or images

it must be simple