[NTG-context] Uppercase Greek in large fontsize

Hi all, I had a problem with typesetting uppercase Greek in large fontsize. I am doing a screen presentation so I want to work with 18pt body font size. But uppercase greek letters all disappear whenever I use anything bigger than 12pt. All small case greek letter are fine, though. Wondering if there is a simple fix to it. Thanks a lot! \setupbodyfont[roman,18pt] \starttext Hello world! $$\delta \Delta \alpha \Alpha \gamma \Gamma $$ \stoptext Best regards, Shen Chen