disable textbox in datalist

Dear all,

I have bound a datalist to some table in a database. Everything works fine; I can delete, edit, and update a record. I am just wondering whether it is possible to disable a textbox defined within the <EditItemTemplate> environment like this:
<EditItemTemplate>

...
   <asp:DropDownList Runat="server" ID = "unit" DataSource = '<%# getValues( "unit" ) %>' DataTextField = "unit_name" DataValueField = "unit_uid" />
...

</EditItemTemplate>

I can 'access' the value of the dropdownlist in my OnUpdate( Object source, DataListCommandEventArgs e ) method like this:

( (DropDownList) (e.Item.FindControl( "unit" )) ).SelectedItem.Value

Unfortunately, I cannot do something like this:

public void OnEdit( Object source, DataListCommandEventArgs e )
{
      variable_details_datalist.EditItemIndex = e.Item.ItemIndex;

      …

      ( (DropDownList) (e.Item.FindControl( "unit" )) ).Enabled = false;

      …
}
            

I obtain a System.NullReferenceException. I don't understand why, as all the EditItems must surely be initialised when the OnEdit handler is called?

Looking forward to your reply.

Many thanks in advance.