[NTG-context] font size > 12pt

Hi, I would like to have font bigger than 12pt. When I try following, commented line causes error message "Math formula deleted: Insufficient symbol fonts." while compiled with luatex. Could you tell me why and how to fix it? Thanks ZS \starttext \setupbodyfont[rm,13pt] Some reaction O\low{2} $\rightarrow$ O\low{3}. %Some reaction\footnote{ O\low{2} $\rightarrow$ O\low{3} }. \stoptext