[NTG-context] index question

I couldn't find anything on the wiki or in the documentation: I have defined a special index which will typeset numbers in bold: \setupregister[index][imp][pagestyle=bold] which I invoke like \index[imp::]{modernity} How can I get a register range to use this special style? I tried something like \startregister[imp][signi][signifie]{\em signifié} but get errors "missing csname." What am i overlooking? Thanks Thomas