Problems getting data from extra table using REST

I don't get data from the "select" columns when I use "archive/dynamic". The URL I use is

  "/archive/dynamic?select=y_produkter.x_hovedgruppe,y_produkter.x_produktnr&$filter=y_produkter.id eq 16"

And the response is {"PrimaryKey":"16","EntityName":"y_produkter"}

Can anyone see what I'm doing wrong? Is there another way of getting data from extra tables?