SumBucket on _bucket_count returns zero

[shemmer (Stefan Hemmer)]

I want to count the number of buckets in an aggregation using ElasticSearch 5.6.2. My documents look like this:

"caseId": "123456",
"timestamp": "2018-02-26T12:28:30Z",
"status": "0"

The state of the last document entry determines the state of a case. My goal is to compute the number of cases in a given state. To that end, I have written the following query statement:

POST foo/foo/_search?size=0
{
  "aggs": {
    "caseID_termsAgg": {
      "terms": {
        "field": "caseId.keyword",
        "size": 5,
        "order": {
          "_term": "desc"
        }
      },
      "aggs": {
        "latest_status": {
          "max": {
            "field": "status"
          }
        },
        "topHits": {
          "top_hits": {
            "_source": "status",
            "size": 1,
            "sort": [
              {
                "timestamp": {
                  "order": "desc"
                }
              }
            ]
          }
        },
        "top_hits_filter": {
          "bucket_selector": {
            "buckets_path": {
              "processStatus": "latest_status"
            },
            "script": "params.processStatus == 1"
          }
        }
      }
    },
    "sum": {
      "sum_bucket": {
        "buckets_path": "caseID_termsAgg._bucket_count"
      }
    }
  }
}

I'm expecting the final aggregation of 'sum' to count the number of buckets in caseID_termsAgg. Instead what ES returns a 0 although there a multiple buckets visible in the query result. I tested replacing '_bucket_count' with 'count' and it correctly sums up the number of documents in each bucket. Can someone tell me what I'm doing wrong here?