Custom Feed not validating

Hi,

we have implemented a custom rss feed (similar to the code provided at http://codex.wordpress.org/Customizing_Feeds) with custom tags. Everything works fine. However, when we tried to validate this feed (on http://validator.w3.org/), we got a meesage saying “Undefined item element”-errors for each custom tag. How will it be possible to define these elements in order to validate the feed correctly?

I would really appreciate it if you could provide me with guidance or help on this issue!

kind regards!