join with unpaired records results in CSV schema change error

given two simple csv files: l.csv: id,bezeichnung,id kompo 1,Hallo,500 2,Blah,20 r.csv id,cbezeichnung 500,Welt the joining columns are 'id' and 'id kompo' So l.csv contains a line 2,Blah,20 for which there is no matching record in file r.csv When I call mlr -c join -f l.csv --lp app_ --ul --rp komp_ -j 'id kompo' -r id r.csv the command fails with the error message: mlr: CSV schema change: first keys "id kompo,app_id,app_bezeichnung,komp_cbezeichnung"; current keys "id,bezeichnung,id kompo" mlr: exiting due to data error. The command succeeds yet doesn't print the non-matching lines when I omit --ul