[NTG-context] t-nath difficulty

I'm taking a look at nath (via t-nath). Here's the very first thing I tried: In the following code, the first formula typesets as I want, and the second doesn't. But the syntax in the second looks correct. Bug or feature? or setup problem? \usemodule[nath] \starttext $ y = (1 + \frac{}{a}{b}) $ $ y = (1 + \frac{a}{b}) $ \stoptext