Another reason this could be happening is if you are using an iframe that has the sandbox attribute and allow-same-origin isn't set e.g.:
// page.html
<iframe id="f" src="http://localhost:8000/iframe.html" sandbox="allow-scripts"></iframe>
<script type="text/javascript">
var f = document.getElementById("f").contentWindow;
// will throw exception
f.postMessage("hello world!", 'http://localhost:8000');
</script>
// iframe.html
<script type="text/javascript">
window.addEventListener("message", function(event) {
console.log(event);
}, false);
</script>
I haven't found a solution other than:
f.postMessage("hello world!", '*');