Possible Duplicate:
problem compiling some 'math' using LuaLaTeX
While trying to write an equation it seems that the square root does not like particular elements. I tried to pinpoint the problem but I am a little stuck. After some time I could get a MWE together to reproduce the problem.
\documentclass[12pt]{article}
\usepackage{lmodern}
\usepackage[intlimits]{amsmath}
\begin{document}
\begin{align}
\tau_{1,2} = \sqrt{ \frac{(k)^2}{4 k_a^2} } \\
\tau_{1,2} = \sqrt{ \frac{(k)^2}{4 k_a} }
\end{align}
\end{document}
The output with lualatex looks as follows: 
There are no error messages in the output. The problem disappears when I either disable \usepackage{lmodern} or switch to pdftex instead of lualatex. Is this a known bug, am I doing something wrong or is there a workaround?