[NTG-context] metafun problem

Dear ConTeXtuals, the following MP-inclusion produces fatal error (! TeX capacity exceeded, sorry [parameter stack size=6000].): starttext \startMPcode label(btex $\underbrace{a}_{b}$ etex,(0,0)); \stopMPcode \stoptext I use mkii, but the code should work. Has anyone an explanation? With many thanks for any hint Albrecht