Haha, well spotted. Yeah, it seems to maybe be an issue of how Google Fonts sends preview text to the URL, and the “+” conflicts with the general separator. I’ve sent a message to the folks over there, and hopefully they will put it in their backlog to fix.
The font definitely has the “+” symbol, along with many other math & currency glyphs (and many more).
Please check it out (and try the preview/tester) at: