[NTG-context] Strange behavior with Utopia fonts in math mode

Hello, I compiled this file with a strange exit for mathematical formula. What's missing in my file ? Thank you. Fabrice \loadtypescriptfile[mathdesign] \usetypescriptfile[utopia] \definetypeface[mainface][rm] [serif][palatino][default][encoding=texnansi] \definetypeface[mainface][ss][sans] [helvetica] [default][encoding=texnansi] [rscale=auto] \definetypeface[mainface][mm] [math] [utopia] [default] [rscale=auto] \setupbodyfont[mainface,12pt] \starttext \input knuth \startformula f(x) = \sum_{i=1}^n a_i x^i \stopformula \stoptext