Go： 无效操作 类型 *map[key]value 不支持索引

我正在try 编写一个函数来修改通过指针传递的原始贴图，但是GO不允许这样做.假设我有一张很大的 map ，不想来回复制.

使用按值传递的代码正在工作，正在做我需要的事情，但涉及到按值传递(playground):

package main

import "fmt"

type Currency string

type Amount struct {
    Currency Currency
    Value float32
}

type Balance map[Currency]float32

func (b Balance) Add(amount Amount) Balance {
    current, ok := b[amount.Currency]
    if ok {
        b[amount.Currency] = current + amount.Value
    } else {
        b[amount.Currency] = amount.Value
    }
    return b
}

func main() {
    b := Balance{Currency("USD"): 100.0}
    b = b.Add(Amount{Currency: Currency("USD"), Value: 5.0})

    fmt.Println("Balance: ", b)
}

但是如果我try 将参数作为指针传递，如下面的(playground):

func (b *Balance) Add(amount Amount) *Balance {
    current, ok := b[amount.Currency]
    if ok {
        b[amount.Currency] = current + amount.Value
    } else {
        b[amount.Currency] = amount.Value
    }
    return b
}

我发现编译错误:

prog.go:15: invalid operation: b[amount.Currency] (type *Balance does not support indexing)
prog.go:17: invalid operation: b[amount.Currency] (type *Balance does not support indexing)
prog.go:19: invalid operation: b[amount.Currency] (type *Balance does not support indexing)

我该怎么处理这件事呢？