Some problems with aggregates and inner filters


I have some problems with creating a dgraph query. I have a scheme:

type Operation {
  id: String
  amount: int
  country: Country

type Country {
  name: String

I want to get a sum of operations where country = ‘UK’ (for example). How i can do this?
When i had written a query like:

query all() {
    var(func: type(Operation)) @cascade {
        amount as amount
        country @filter(eq(name, "UK"))

    aggregates() {
       sum : sum(val(amount))

I got the sum of total operations, but when I had just done a query with similar parameters (without aggregation) I got a correct list of operations. Suggest me, please, how can I get a correct sum of operations?