Can't prevent from merge

[lizhen2017]

a: {
    1: 1
}
b: {
    index: ${a}
}
c: ${b} {
    index: null
    index: {
        2: 2
    }
}

{ "index": { "2": 2 } } is expected, but output is { "index": { "1": 1, "2": 2 } }