[NTG-context] underbar in MP text

Hi all, if \underbar is used inside a MP text, it always uses a fixed rule thickness, regardless of the settings in ConTeXt. How can I change this? TIA, Peter \setupoutput[pdftex] \setMPtext{testtext} {} \startusableMPgraphic{mptext} draw textext.rt(\MPstring{testtext}) ; \stopusableMPgraphic \starttext \setupunderbar[rulethickness=2pt] \underbar{Thick line} \setMPtext{testtext}{\underbar{Thick line}} \useMPgraphic{mptext} \setupunderbar[rulethickness=.15pt] \underbar{Thin line} \setMPtext{testtext}{\underbar{Thin line}} \useMPgraphic{mptext} \stoptext