Solr indexing: date field with (empty) no value

I have the following field definition in my SOlr Schema

name="DvdReleaseDate"   type="date "  stored="true" required="false"

My input xml file contains movie names with DVDReleaseDate. So for brand new releases, this field in empty (aka null) Now when I am indexing, SOLR was throwing empty value exception.

What are my other alternatives?

Basically, I need to support queries based on DVDRelease date & at the same time, empty values are valid use case.

Editing my question based on feedback:

I have following in the solr schema

field name="dvdReleaseDate" type="date" stored="true" required="false"

Exception details are

HTTP ERROR 400 Problem accessing solr update. Reason: Invalid Date String:''

Sep 8, 2011 9:14:22 AM org.apache.solr.common.SolrException log
SEVERE: org.apache.solr.common.SolrException: Invalid Date String:''
   at org.apache.solr.schema.DateField.parseMath(DateField.j开发者_如何学Pythonava:165)

    at org.apache.solr.schema.TrieDateField.createField(TrieDateField.java:169)
    at org.apache.solr.schema.SchemaField.createField(SchemaField.java:98)
    at org.apache.solr.update.DocumentBuilder.addField(DocumentBuilder.java:204)
    at org.apache.solr.update.DocumentBuilder.toDocument(DocumentBuilder.java:277)
    at org.apache.solr.update.processor.RunUpdateProcessor.processAdd(RunUpdateProcessorFactory.java:60)
    at org.apache.solr.handler.XMLLoader.processUpdate(XMLLoader.java:147)
    at org.apache.solr.handler.XMLLoader.load(XMLLoader.java:77)
    at org.apache.solr.handler.ContentStreamHandlerBase.handleRequestBody(ContentStreamHandlerBase.java:55)
    at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:129)
    at org.apache.solr.core.SolrCore.execute(SolrCore.java:1360)
    at org.apache.solr.servlet.SolrDispatchFilter.execute(SolrDispatchFilter.java:356)

When adding the document without dvdReleaseDate, don't include the field in the XML document. E.g. instead of:

<add>
  <doc>
    <field name="id">123</field>
    <field name="dvdReleaseDate"></field>
  </doc>
</add>

do:

<add>
  <doc>
    <field name="id">123</field>
  </doc>
</add>