[NTG-context] Background color with \showgrid and columns

Dear list, When processing the attached mwe: \setuplayout[grid=yes] \showgrid \setupcolumns[n=2] \starttext \startcolumns \input knuth \column \input knuth \stopcolumns \stoptext in the final result there is a background color. How do I get rid of it? Marcus Vinicius