Is an empty string a valid JSON key?

Is a JSON object having an empty string as a key valid, for example { "c": "x", "": "y" }?

It certainly seems to be, or at least it doesn't seem to break the web if I using it in a browser as per this jsfiddle having the following code:

var a = { "a": "x", "b": "y" };
var b = { "c": "x", "": "y" };
var c = JSON.stringify(a);
var d = JSON.stringify(b);
console.log(c);
console.log(d);
var e = JSON.parse(c);
var f = JSON.parse(d);
console.log(e);
console.log(f);