[AccessD] A2K: Can't update (ADO) Recordset

Hello all

Can anyone tell me why I get the following Error message when I try
and  update the following record set?
=================================================
Error Number 3251
Current Recordset does not support updating.
This may be a limitation of the provider, or of the 
selected locktype
=================================================

Code...
Dim con As Object
Dim rs As Object
Dim sel_SQL As String

Set con = Application.CurrentProject.Connection

Set rs = CreateObject("ADODB.Recordset")


sel_SQL = "SELECT tblMyTable.*, tblMyTable.FieldToBeUpdated FROM tblMyTable" _
                & "WHERE (((tblMyTable.FieldToBeUpdated ) Is Not Null));

rs.Open sel_SQL, con, 1 '<====Error line is here somewhere me thinks
    
    With rs
        While Not .EOF
            !FieldToBeUpdated = -1
            .MoveNext
        Wend
    End With