I guess you also have something like this in the manifest:
Code:
<item id="cover" href="Cover.xhtml" media-type="application/xhtml+xml"/>
as otherwise the bit in the spine would be pointing to a non-existing idref. In that case, you could try pointing the guide element to Cover.xhtml, instead of the jpg file. That would make it ePub compliant, but it might not convert nicely to Kindle, you'd have to try.