400. That’s an error.

Google cannot redirect you to http://worldcat.org/search?q=Isidori+mercatoris+decretalium+collectio+post+ed.+principem+ad+prelum+nunc+primum+revocata,+mendis+repurgata+[&c.].+Accedunt+prolegomena+amplissima+cura+et+studio+H.+Denzinger+isidorus+(mercator.) because the request was invalid.

This might be because the link was created a long time ago. Try your search again on Google. That’s all we know.