$project or $group does not support <document>

I'm trying to run aggregate with projection but i get NotSupportedException: $project or $group does not support <document>. I am running version 2.4.4 of driver with mongodb v3.4.

var filter = Builders<T>.Filter.Regex(x=>x.Value,"/test/gi");

var aggregate = collection.Aggregate()
       .Match(filter)
       .Project(x => new 
       {
          Idx = x.Value.IndexOf("test"),
          Result = x
       })
       .SortBy(x => x.Idx);

I thought IndexOfCP is supported.

What am i doing wrong here?