[AccessD] A2K:Change of Value Not working

Hello all
I am getting the "You can't assign a value to this object" error with the following code

    If Me.chkRequestorsToNote <> 0 Then
        Me.chkRequestorsSeen = -1   <====Error on this line
    Else
        'do nothing
    End If

Both Check boxes exist on the form. 
Both are bound to fields that exist in the underlying table
Yet when I make the checkboxes unbound this code works.
I have also tried the dot value property as well eg  Me.chkRequestorsSeen.Value = -1
Still no good.

What am I missing?

Many thanks in advance

Darren