[NTG-context] Metafun problem

All, I've got the context minimals installed and I'm compiling the following (from the new Metafun manual) using MKIV: \startuniqueMPgraphic{name} path p; p:=unitsquare xscaled OverlayWidth yscaled OverlayHeight; fill p withcolor .625yellow; draw p withcolor .625red; \stopuniqueMPgraphic \starttext Hello, World! \defineoverlay[my graphic][\uniqueMPgraphic{name}] \button[background=my graphic,frame=off]{Go Home}[firstpage] \stoptext No button shows up. Am I missing something? David