Handle Null Value For DropDownList?

Using asp.net 2.0 and visual studio 2005. The question is regarding the following ER diagram:

 
I've made Firstname, lastname, buildingID and RoomNum all required fields. I've got a modified GridView that displays all of the table Faculty columns.  It's been modified so the BuildingID and DepID are resolved to their actual field names and displayed in a DropDownList.  In the dropdown list I used for inserting (a seperate DetailsView control), I manually inserted an item into the Department dropdownlist which had the text "-- Select a Department --" with a value of -1.  MS SQL didn't like that -1 value so I wrote the following code to fix it:  protected void dsFaculty_Inserting(object sender, SqlDataSourceCommandEventArgs e)
{
if (e.Command.Parameters["@DeptID"].Value.ToString() == "-1")
{
e.Command.Parameters["@DeptID"].Value = null;
}
}
 That means of course DeptID is null, which is ok.  The problem arises is when I try to edit that row in the GridView.  I get the an error 'ddlDepartment' has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value
Ideally, I'd like to make the dropdown list in the GridView show "-- None --" for the DeptID if it comes across a null value.  I already tried playing around with the Command.Parameters in the dsFaculty_Selected function, but it didn't work.  Ideas?