Maybe you have noticed that since version 2014 there is a limitation to set the minimum pipe length. You can only set a value which is equal or greater than 5. Otherwise you will get this message:
You can bypass this limitation by setting the project variable MINPIPELENGTH in the database.
Therefore open the piping.dcf (SQLite project) respectively the <prefix>piping-database (SQL Server project) and switch to the table "PnPProjectVariables". Set a value less than 5 to MINPIPELENGTH. Afterwards you have to re-open the project.