DBRef breaks in the shell when _id is 0, or null, or other valid _ids that happen to evaluate to false:

> new DBRef("a", 0).fetch()
assert failed : need an id
Error("Printing Stack Trace")@:0
()@shell/utils.js:35
("assert failed : need an id")@shell/utils.js:46
(0,"need an id")@shell/utils.js:54
()@shell/utils.js:572
@(shell):1

In shell/utils.js, DBRef.prototype.fetch should be:

assert(typeof(this.ns) !== 'undefined');
assert(typeof(this.id) !== 'undefined');