You have right I have the same with textarea.
To fix this you can add IsNull(o) function to your webresource:
IsNull(o) {return "undefined" == typeof o || "unknown" == typeof o || null == o}
or add a reference to global.ashx:
<SCRIPT type=text/javascript src="/_common/global.ashx"></SCRIPT>
I like this error: IsNull is null or undefined
Lol