Summary

Hi,

when i try to GroupBy() ,then OrderBy(Count()) i get a MongoDB.Driver.MongoCommandException: Command aggregate failed: PlanExecutor error during aggregation :: caused by :: The argument to $size must be an array, but was of type: missing.'

How to Reproduce

class Example {
 string Name
}

Collection
.AsQueryable()
.GroupBy(x => x.Name)
.OrderBy(x => x.Count())

MongoDB.Driver.MongoCommandException: 'Command aggregate failed: PlanExecutor error during aggregation :: caused by :: The argument to $size must be an array, but was of type: missing.'