TypeError: decl.init.body is undefined

Created by: Jonathan50

When I enter

syntax fn = function (ctx) {
  let args = ctx.next().value;
  let body = ctx.next().value; 
  return #`function ${args} ${body}`;
}

(fn () {
  return 42;
})

into the online editor I get this error message:

TypeError: decl.init.body is undefined

I'm using Firefox Nightly version 49.0a1.