iassign does not work correctly with ES6 Map about immutableassign HOT 2 CLOSED

// Works
// Assume copyFunc option is set

var nested1 = { a: { b: new Map() } };
nested1.a.b.set("c", { "d": 100 })

var nested2 = iassign(
    nested1,
    (m, ctx) => m.a.b,
    (b) => {
        b.set(0, 'first'); return b;
    }
);

// Will not work
// Assume copyFunc option is set

var nested1 = { a: { b: new Map() } };
nested1.a.b.set("c", { "d": 100 })

var nested2 = iassign(
    nested1,
    (m, ctx) => m.a.b.get("c"),
    (c) => {
        c.e = 200; 
        return c;
    }
);