\startuniqueMPgraphic{column:bg}
	fill OverlayBox withcolor yellow;
\stopuniqueMPgraphic
\defineoverlay[columnbg][\uniqueMPgraphic{column:bg}]


\definecolumnset[TOC][n=4]
%\setupcolumnset[TOC][background=columnbg]% works ok
\setupcolumnset[TOC][1][background=columnbg]% any indexed setup fails
% in my real world example the complete third argument is printed instead


\starttext
\startcolumnset[TOC]
\input tufte.tex
\stopcolumnset
\stoptext