value can not be null in datatable in vb.net 2005

Hi,

I have the following code: -

SQL = "Select * from dbo.Locations"
        dbOpen()
        Dim da As New SqlClient.SqlDataAdapter(SQL, oConn)

        da.Fill(ds, "Locations")
        DataGridView1.DataSource = ds.Tables("Locations")

It always comes up with: -

Value cannot be null.
Parameter name: dataSet

What does this mean please and how to I resolve it.

Many Thanks
Lee