There's a bug in the externalize and internalize prelude functions. They both access the first struct field before checking if there's no fields (here and here)... here's a simple demo which throws a JS exception Cannot read property 'prop' of undefined:

https://play.jsgo.io/308c189e795b573511cbc4d4606af6b0a60a065a

I've fixed it here: #824

p.s. I don't have node so I didn't run the uglify script... Perhaps someone can do that for me?