[NTG-context] UTF-8 and Sigma

Hello, using \enableregime[utf] \starttext Σ, σ or ς? \stoptext with pdftex prints "Σ, σ or σ?" (but the latter should be a \varsigma and not a \sigma). And with LuaTeX the result is even "Σ, or ?", i.e. the small sigmas disappear completely. Can this be fixed, even though the proper way would be to use a different font? Tobias