I ran into this one when trying to use Go functions that are callable from JavaScript code.

Playground link:
http://www.gopherjs.org/playground/#/A_bP7IWgxi

Basically, $internalize has no code path for 'undefined' values that are mapped to interfaces.

I would like to recommend the following fix, for line 245 in compiler/prelude/jsmapping.go like so:

    if (v === null || v === undefined) {
      return $ifaceNil;
    }

This changes $internalize such that undefined is converted to nil, just like null, for interfaces.