Elasticsearch Bucket sort on nested field - elasticsearch

Question

I have a problem with a bucket sort on nested field on Elastic 7.1.0:
My index has the following mapping:
{
"mapping": {
"dynamic": "strict",
"properties": {
"created_at_timestamp": {
"type": "date"
},
"url": {
"type": "keyword",
},
"title": {
"type": "keyword",
},
"entities": {
"type": "nested",
"properties": {
"counter": {
"type": "long"
},
"metric": {
"type": "long"
},
"id": {
"type": "long"
},
"relevance": {
"type": "float"
},
"weighted_metric": {
"type": "float"
}
}
}
}
}
}
and I need to order this documents by "weighted_metric", filtered for a specific entity id. I wrote this query:
GET my_index/_search?size=0
{
"query": {
"bool": {
"must": [
{
"nested": {
"path": "entities",
"query": {
"term": {
"entities.id": "27374"
}
}
}
}
],
"must_not": [
{
"term": {
"title": {
"value": ""
}
}
}
]
}
},
"aggs": {
"by_url_and_title": {
"composite": {
"sources": [
{
"final_url": {
"terms": {
"field": "final_url"
}
}
},
{
"title": {
"terms": {
"field": "title"
}
}
}
]
},
"aggs": {
"sum_metric": {
"nested": {
"path": "entities"
},
"aggs": {
"weightedmetric": {
"filters": {
"filters": {
"new": {
"bool": {
"should": [
{
"term": {
"entities.id": "27374"
}
}
]
}
}
}
},
"aggs": {
"wmetric": {
"sum": {
"field": "entities.weighted_metric"
}
}
}
},
"w_sort": {
"bucket_sort": {
"sort": [
{
"weightedmetric.wmetric": {
"order": "desc"
}
}
],
"size": 10
}
}
}
}
}
}
}
}
And I have this error:
{
"error": {
"root_cause": [],
"type": "search_phase_execution_exception",
"reason": "",
"phase": "fetch",
"grouped": true,
"failed_shards": [],
"caused_by": {
"type": "class_cast_exception",
"reason": "org.elasticsearch.search.aggregations.bucket.nested.InternalNested cannot be cast to org.elasticsearch.search.aggregations.InternalMultiBucketAggregation"
}
},
"status": 503
}
If I don't try to order the buckets everything works fine.
Can someone help me with this query? I need to order the buckets by weighted_metric. thanks