[NTG-context] reference error

Hallo, I must do someting wrong --- \starttext \startchapter [ title={Vorwort}, reference=chap:vorwort ] Siehe \in{Kapitel}[chap:neu] \at{Seite}[chap:neu]. \stopchapter \startchapter [ title={Neu}, reference=chap:neu ] Bla. \stopchapter \stoptext --- I get an error: unknown reference '[][chap:neu]' Wolfgang