custom string search on transaction not working

I have a custom string field on transacactions and I'm trying to do a TransactioNSearchBasic to return all matches with a EMPTY custom field value.  The filter is having no effect.

Anyone have any ideas?  Known issues?

Here is the soap:

 <soap:Envelope xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">      <soap:Header>          <searchPreferences xmlns="urn:messages_2_6.platform.webservices.netsuite.com"/>      </soap:Header>      <soap:Body>          <search xmlns="urn:messages_2_6.platform.webservices.netsuite.com">              <searchRecord xsi:type="q1:TransactionSearchBasic" xmlns:q1="urn:common_2_6.platform.webservices.netsuite.com">                  <q1:department operator="anyOf">                      <searchValue internalId="2" xmlns="urn:core_2_6.platform.webservices.netsuite.com"/>                  </q1:department>                  <q1:status operator="anyOf">                      <searchValue xmlns="urn:core_2_6.platform.webservices.netsuite.com">_returnAuthorizationPendingReceipt</searchValue>                  </q1:status>                  <q1:type operator="anyOf">                      <searchValue xmlns="urn:core_2_6.platform.webservices.netsuite.com">_returnAuthorization</searchValue>                  </q1:type>                  <q1:customFieldList>                      <customField operator="empty" internalId="custbody_rm_return_postage_track_num" xsi:type="SearchStringCustomField" xmlns="urn:core_2_6.platform.webservices.netsuite.com"/>                  </q1:customFieldList>              </searchRecord>          </search>      </soap:Body>  </soap:Envelope>    //  Here is a single record from the response, as you can see the field I'm trying to filter out isn't included (because it's empty) yet the record is returned    <ns2:record xsi:type="ns14:ReturnAuthorization" internalId="12107" xmlns:ns14="urn:customers_2_6.transactions.webservices.netsuite.com">      <ns14:createdDate>2008-06-25T10:14:00.000-07:00</ns14:createdDate>      <ns14:lastModifiedDate>2008-06-25T10:22:00.000-07:00</ns14:lastModifiedDate>      <ns14:entity internalId="1078">          <ns2:name>, Nina - ****</ns2:name>      </ns14:entity>      <ns14:tranDate>2008-06-25T00:00:00.000-07:00</ns14:tranDate>      <ns14:tranId>76</ns14:tranId>      <ns14:department internalId="2">          <ns2:name>Retail Sales</ns2:name>      </ns14:department>      <ns14:location internalId="13">          <ns2:name>PMD Alton : PMD-BOND : Returned Product</ns2:name>      </ns14:location>      <ns14:partner internalId="906">