guid must be a full URL, unless isPermaLink attribute is false

By default, the guid element specifies a permanent link for an item, the value must be a full URL (starting with ""http://"").

If you are using the guid element as a permanent link, make sure the value is a full URL, including the "http://" at the beginning.

If you are using the guid element simply as a unique identifier (and not a link to the item), then the value can be whatever you want, but you must include the attribute isPermaLink="false" in the guid tag.

Both of these are valid guids:

<guid>http://www.example.com/archives/000054.html</guid>

<guid isPermaLink="false">article 54 at example.com</guid>