reduce函数给出“Uncaught TypeError:无法设置未定义的属性'title'”

我尝试使用reduce函数在javascript中重塑JSON对象。

ac['new_object'] = {}; . 我这样做,所以我不明白为什么我会看到错误: Uncaught TypeError: Cannot set property 'title' of undefined

my_json = JSON.parse('[{"key":"id","val":{"boost":4}},{"key":"title","val":{"boost":4}},{"key":"plot","val":{"boost":3}},{"key":"actor","val":{"boost":2}}]')
console.log(
  field_list.reduce(function(ac, cu){
    ac[cu.key] = {}
    ac[cu.key] = cu.val
  }, {})
)

如何正确地减少这个JSON对象?我犯了什么错?

目标格式:

{
    tales: {
        foo: {"id": 2},
        bar: {"title": 1},
        ...
    }
}