xmllint and relax ng validation

Hi

I'm trying to validate my document using xmllint but get this error:


 xmllint --noout --valid --relaxng
/usr/share/xml/docbook5/schema/rng/5.0/docbook.rng main.xml
main.xml:2: validity error : Validation failed: no DTD found !
/docbook" xmlns:xi="http://www.w3.org/2001/XInclude" version="5.0" xml:lang="en"

I thought that docbook 5 didn't require a dtd?