I am trying new feature on mongo-driver for `$function` , here is my syntax

function := fmt.Sprintf("function(p) { return p; }")
addFunc := bson.D{
   {"$addFields", bson.D{
      {"addFunc", bson.D{
         {"$function", bson.D{
            {"body", function},
            {"args", bson.A{"yes"}},
            {"lang", "js"},
         }},
      }},
   }},
}

I use that addFunc init my aggregate pipeline, and it took long to proses if add that `addFunc` into my pipeline aggregate, and at the end, I got error "must not be null" , I don't know what's wrong with it, is that any idea for my case? or mongodb go not support $function yet??
I am using latest mongo atlas