[NTG-context] Missing { inserted using placefigure and postponing

Hi, I get a “Missing { inserted.” error under certain circumstances. Here is an example: \starttext \blackrule [height=8cm] \startpostponing \placefigure [] {foo}{bar} \stoppostponing \blackrule [height=15cm] \startlines a b \stoplines \stoptext I used black rules to insert boxes of a particular height, any content would do. Funnily enough it works with \placefigure [] {foo}{bar}, but not with \placefigure [] {foo}{bar} I have no clue what's wrong here. Marco