'Unspecified error' in IE

I'm working on an old project (fixing issues but doing as few changes as possible). There are several pages which use FCKEditor, and one of the pages is causing a js error for our client but not us (both usingIE8). The error is 'unspecified error' on line 18 of fckeditorcode_ie_1. The problem code seems to be:

return documentElement.createStyleSheet(cssFileUrl);

Are there any suggestions? I can't debug it myself, as the error only seems to be happening for our client.

Thanks for any help provided.