[NTG-context] (Mathe)Problem with $\tilde x$ and $\bar x$

Hello, the example further down works fine with the latex standard fonts and e.g. eulervm, but not with fourier. Could somebody help me? Thank you very much in advance, Helmut % ----------example------------- \setupoutput[pdftex] \usetypescript[fourier][ec] \setupbodyfont[fourier,12pt] \starttext $\tilde x$ ~~=~~ Median, Zentralwert \crlf $\bar x$ ~~=~~ Arithmetischer Mittelwert \stoptext