[NTG-context] Setup font size by dimension variable

Hi all, I want to set the font size by using the dimension variable, but it doesn't work. What am I doing wrong? \definemeasure[AAA][20pt] \newdimen\BBB \BBB=20pt \starttext TEXT % don't work \switchtobodyfont[\measure{AAA}] AAA % don't work \switchtobodyfont[\the\BBB] BBB % ok \switchtobodyfont[20pt] TEXT 20pt \stoptext -- Dmitriy