I have found out some more information about my problem... The JSF element that is being validated by the validator, is located in a tag file from a tag library that I have written. The tag file is referring to: value="#{searchInfo.fullText}" "searchInfo" is actually a parameter that is passed to the tag file from the underlying page. It seems to be this reference to "searchInfo" that is causing an IllegalArgumentException within seam. (The exception seems to be caused because modelInstance evaluates to null within the validate method in org.jboss.seam.core.Expressions) If I refactor the code, and take all the elements out of the tagfile and put them into the main page, then the validation works correctly. Richard. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4054384#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...