I am trying to get the STIX Two fonts to display oldstyle numerals. 

I have the oldstyle numerals in STIX Two Text, the problem is the math font.

So far, my mwe is:
\definefontfamily[mainface][rm][STIX Two Text][] %onum=yes does not seem to work
\definefontfamily[mainface][mm][STIX Two Math][]
\definefontfeature[f:oldstyle][onum=yes]

\setupbodyfont[mainface, 18pt]
\addfeature{f:oldstyle}

\starttext
123456789
\blank[big]
$\sqrt{2} + 1234$
\stoptext

Any pointers will be greatly appreciated.

Alan