Getting handle on ListBox inside datagrid

[Sosh]

Hi,

I'm having a bit of a job getting a handle on a ListBox that i've
placed in one column of the footer in a DataGrid.

Here's what I've tried so far:

- Declaring a protected listbox (with the same id as the aspx) in code
behind, and accessing that directly.

- using (ListBox)FindControl("IDoflistbox")

- using (ListBox)IDofdatagrid.FindControl("IDoflistbox")

However, in each case I get an 'Object reference not set to an
instance of an object' error when I try and do anything with the
control.

Could anyone tell me what might be wrong here?

Thanks