[NTG-context] Strange behavior with square brackets in formula and \rm subscripts

I just found out that I cannot (with MkIV on Windows XP, ConTeXt ver. 2011.02.09 16:42) compile the following \starttext This formula produces an error: \startformula [r_{\rm test}]=a \stopformula \stoptext \starttext However, this works: \startformula a=[r_{\rm test}] \stopformula What's going on here? Using \mathrm instead of \rm in the subscript is compiling without complaint, but produces the wrong output. Julian