Use $$value with C# drivers - Use of undefined variable error


I have a problem with aggregation pipeline using C# drivers.
I am trying to build Project() stage and use $reduce inside (trying to rewrite example from Question no. 3 from Final Exam in Aggregation Framework).

The problem is when I am trying to use $$value inside BsonArray.

Such an error appeared:

Besides this little part of aggregation pipeline other steps of the pipeline works well in insolation.

Did anyone encounter such problem before ? Am I doing something wrong?