[NTG-context] "Missing number, case 7, treated as zero" error message.

Dear lister, I have an error which I can not locate. Please help me what is wrong in the following example. Thanks for reading. Regards, Dalyoung \useMPlibrary[dum] \starttext \startplacefigure[location=,reference=fig:gateNOT,title=] \startcombination[nx=2,ny=1] {externalfigure[dummy][width=.25\textwidth]}{a} {externalfigure[dummy][width=.25\textwidth]}{b} \stopcombination \stopplacefigure \stoptext Log file: metapost > library 'dum' is loaded fonts > preloading latin modern fonts (second stage) fonts > 'fallback modern rm 12pt' is loaded tex error > tex error on line 5 in file /Users/graph/Library/Mobile Documents/com~apple~CloudDocs/Documents/test.tex: Missing number, case 7, treated as zero <to be read again> ] {externalfigure[dummy][width=.25\textwidth] }{a} 1 \useMPlibrary[dum] 2 \starttext 3 \startplacefigure[location=,reference=fig:gateNOT,title=] 4 \startcombination[nx=2,ny=1] 5 >> {externalfigure[dummy][width=.25\textwidth]}{a} 6 {externalfigure[dummy][width=.25\textwidth]}{b} 7 \stopcombination 8 \stopplacefigure