[NTG-context] factor=fit fails

Strange error with \externalfigure \starttext \vbox to \textheight{% \vfill Foo\ \vfill \centerline{\externalfigure[cow][width=0.5\textwidth,height=0.5\textheight,factor=fit ]} \vfill } \stoptext ! Missing number, treated as zero. system > tex > error on line 1 in file test.tex: Missing number, treated as zero ... 1 >> \starttext 2 \vbox to \textheight{% 3 \vfill 4 Foo\ 5 \vfill 6 \centerline{\externalfigure[cow][width=0.5\textwidth,height=0.5\textheight,factor=fit ]} 7 \vfill 8 } 9 \stoptext 10 <to be read again> f \p_factor ->f it -- luigi