[flexcoders] Error #1034: Type Coercion failed: cannot convert "" to Date w/ Datagrid

[timgerr]

Hello all, 
I am trying to add a datefield to a datagrid using a itemEditor.  I
have seen that people are having this problem but I canot find a
solution (That I understand).  Has anyone ever had this problem and
what did you do to solve it.  Here is my code:


<mx:DataGrid id="CertDG" width="528" height="60%" editable="true">
   <mx:columns>
    <mx:DataGridColumn headerText="Name" dataField="cert_name"/>
    <mx:DataGridColumn headerText="Issuer" dataField="issuer"/>
    <mx:DataGridColumn headerText="Achieved"
dataField="date_achieved" editorDataField="text">
<mx:itemEditor>  
<mx:Component>
  <mx:DateField  formatString="DD-MM-YY"
yearNavigationEnabled="true"    toolTip="Format:DD-MM-YY"/>                     
                 
</mx:Component>                                          </mx:itemEditor>
</mx:DataGridColumn>
</mx:columns>
</mx:DataGrid>


Thanks for the read, 
timgerr