WARNING: cannot add @xml:base to node set root element. Relative paths may not work.

Hello,

since a while I get the following warning during the transformation with the docbook distribution:

WARNING: cannot add @xml:base to node set root element.  Relative paths may not work.

I'm not sure what triggers this warning, a Google search resulted in hints about Docbook V5.0 but I'm still using Docbook 4.5.

I'm currently using the docbook-xsl-1.77.1 distribution and I think (but I'm not sure) that I have this warning since I upgraded from 1.76. The output looks ok to me, but this warning does not create a warm fussy feeling. ;-)

Any hint how to get rid of this warning? 

Best regards, Lars