xsd:date and empty element!

Hi,
i've put in my Schema something like that:

     <xsd:element name="iss_date" type="xsd:date" minOccurs="0"/>

and in my XML file:

<iss_date/>

and i get the following message with XMLSPY 4.3: " invalid value for 
datatype date in element 'iss_date'  ".

somebody knows what's wrong with it ?

Thank you....othman