[fw-formats] Zend_Search_Lucene keyword search issue

Hi,
I'm having a problem while trying to find a document with certain
keyword. I'm indexing my document like this:

$doc_id = md5($xml_doc->id);
$lucene_doc->addField(Zend_Search_Lucene_Field::Keyword('document_id',
$doc_id));
...
$this->index->addDocument($lucene_doc);

the document is added fine, but the problem I have is that when I try to
find an already indexed document in order to update it(that is, delete
it and add it again to the index), I get no results. The curious thing
is that if I open the generated index with Luke(a tool to browse lucene
indexes among other things: http://www.getopt.org/luke/) and I run the
same search query, I get the result I was waiting for...
Anyone experiencing the same problem?

Thanks in advance

Regards