How To Insert A Zero Length String Into A Field Using SqlDataSource?

I have a few columns in table with default value defined as zero length string (''). I want to insert record from DetailsView which uses SqlDataSource as DataSource. In the ItemInserting event, if the data is not valid, I want to use zero length string for the column. But I always get Null instead of zero length string. The code in ItemInserting event looks like this:

If objddl.SelectedIndex > 0 Then
        e.Values("myFld") = objddl.SelectedItem.Value
Else
        e.Values("myFld") = ""
End If

The line: e.Values("myFld") = "" put Null in the column.

How can I set a column as zero length string using the SqlDataSource?

Any help is appreciated.

Thanks.