I believe your problem lies in trying to "update" an existing document with only two fields. Which version of Solr are you using? Partial updates are only supported for Solr 4.x.
Here's some information on what partial updates are and how to do them: http://wiki.apache.org/solr/Atomic_Updates
http://solr.pl/en/2012/07/09/solr-4-0-partial-documents-update/