Properties such as firstChild, nextSibling, etc. are read only and should not be changed, although technically it is possible to do so. This was a conscious decision because PHP does not allow the stacking of method calls on a single line (this has changed in PHP 5, but for the sake of backwards compatability will not change in DOMIT!) like:

$myCD = $cdCollection->getDocumentElement()->getChildNodes(1);

Always modify nodes using appendChild, insertBefore, removeChild, and replaceChild!

There is limited validation in the current version of DOMIT! Try to ensure instead that your code is sound and that any XML passed to the DOMIT! parser is well-formed and valid.