Significant Terms Aggs: bg_count equals zero

Elastic Stack Elasticsearch
1

I am running a very basic significant_terms query as below, and the bg_count is coming out zero no matter what; any ideas?

ES 1.7.2

Query:

{
  "aggs": {
    "trendingStuff": {
      "significant_terms": {
        "field": "description"
      }
    }
  }
}

Response:

    "aggregations": {
        "trendingStuff": {
          "doc_count": 2452,
          "buckets": [
            {
              "key": "random",
              "doc_count": 2449,
              "score": 5908.514169609521,
              "bg_count": 0
2

I knew what was happening; "description" is a nested field, so field name is "doc.description" instead of just "description".