Why not support yield undefind?

[guileen]

I offen get this error

Error: yield a function, promise, generator, array, or object?

And it is hard to trace the problem, mostly it is happen when I yield a undefined value, so why not just allow to yield undefind.

Below code explain a case that how it happen.

function load(x){
   return function(callback) {
     db.load(x, callback);
   }
}
function loadFields(obj) {
   if(!obj) return;
   obj.x = load(obj.x);
   obj.y = load(obj.y);
   obj.z = load(obj.z);
   return obj;
}

co(function*() {
  var obj = yield getFromDb(id) // should return {x:1, y: 2, z:3};
  obj = yield loadFields(obj); // If obj is undefind, this line will occurs a Error, and hard to trace to this line.
})()

[juliangruber]

you should rather check for obj:

var obj = yield getFromDb(id);
if (obj) obj = yield loadFields(obj); 

The way it currently is you at least know where something's wrong, swalling the error is way more obscure

[tj]

it would be much harder to track down if we did allow yield undefined, I'd probably only use object/array support in the co call itself, otherwise it's a little unclear what the intent is since it's such a sugary feature