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