System.ArgumentException
 HResult=0x80070057
 Message=The input type to stage[1] was expected to be MongoDB.Bson.BsonDocument, but was MongoIssue.TestModel_Unwound.
Parametername: stages
 Source=MongoDB.Driver
 StackTrace:
 at MongoDB.Driver.PipelineStagePipelineDefinition`2.VerifyStages(List`1 stages)
 at MongoDB.Driver.PipelineStagePipelineDefinition`2..ctor(IEnumerable`1 stages, IBsonSerializer`1 outputSerializer)
 at MongoDB.Driver.AggregateFluent`2.ThenBy(SortDefinition`1 newSort)
 at MongoDB.Driver.IAggregateFluentExtensions.ThenBy[TResult](IOrderedAggregateFluent`1 aggregate, Expression`1 field)
 at MongoIssue.Program.Main(String[] args) in C:\Users\fals\source\repos\MongoIssue\MongoIssue\Program.cs:line 33

var query = collection.Aggregate()
  .Unwind(x => x.Items)
  .As<TestModel_Unwound>()
  .SortBy(sortExpression1)
  .ThenBy(sortExpression2);