query the brokerdb on web8.5

Hi all,  
I'm querying the brokerdb on Web8.5:  
   CustomMetaValueCriteria index = new CustomMetaValueCriteria(new CustomMetaKeyCriteria("index"), "yes", Criteria.Equal);
   Query myQuery = new Query(index);
   string[] itemUris = myQuery.ExecuteQuery();
Custom metadatavalues in the db are ok (1 record, key_name='index',key_string_value='yes'), but the query with index="yes" or index="no" gives the same results (1). When I change index="yes" to index="something" no results are returned. Any clues?