Cannot write superscripts and subscripts in ConTeXt

Here is my mwe:

\setupbodyfont[gentium,12pt]
\mainlanguage [gr]

\starttext

text in greek , $\alpha^{i}_{j}$

\stoptext


I compile my document using ConTeXt(LuaTeX) and I get the following error:

Math error: parameter \Umathsup_shift_up\textstyle is not set

This problem appears (so far) when I try to use a subscript/superscript in my equations

gentium is just a serif typeface. To use math, you need to define a math typeface as well. I am not sure which math font looks the best with Gentium; I am using Latin Modern just to show how to get rid of the error message.

\definetypeface [mainface] [rm] [serif] [gentium][default]
\definetypeface [mainface] [ss] [sans]  [modern] [default]
\definetypeface [mainface] [tt] [mono]  [modern] [default]
\definetypeface [mainface] [mm] [math]  [modern] [default]

\setupbodyfont[mainface,12pt]
\mainlanguage [gr]

\starttext

text in greek, $\alpha^{i}_{j}$

\stoptext

• I believe TeX Gyre Pagella fits better with Gentium font – Yorgos Oct 24 '16 at 6:57
• To use pagella as the math font, replace the last line with \definetypeface [mainface] [mm] [math] [pagella] [default] – Aditya Oct 24 '16 at 17:25