Aggregations: Value Count Agg cannot be used for sort order

[colings86]

If a value count agg is used for the sort column of a terms aggregation an exception is thrown stating:

ReduceSearchPhaseException[Failed to execute phase [query], [reduce] ]; nested: ElasticsearchIllegalArgumentException[Invalid order path [grades_count]. Missing value key in [grades_count] which refers to a multi-value metric aggregation]; 

Marvel commands to reproduce error: https://gist.github.com/colings86/4dfcb7de6c474ae69c32

Setting order to:

"order": {
    "grades_count": "desc"
}

produces the following error:

{
   "error": "ReduceSearchPhaseException[Failed to execute phase [fetch], [reduce] ]; nested: ClassCastException[org.elasticsearch.search.aggregations.metrics.valuecount.InternalValueCount cannot be cast to org.elasticsearch.search.aggregations.metrics.InternalNumericMetricsAggregation$MultiValue]; ",
   "status": 503
}