Modules & VBA :: Testing Combo Box Null Value

Windows vista
Access 2007

I can't seem to figure out how to test a combo box null value.I try:

Code:
If IsNull(Me.cboManufacturer) Then
MsgBox ("Please select category")
Else
DoCmd.OpenReport "rptItemCategory", acViewPreview
End If

This works if the combo box IS NOT NULL, but if null it gives me:

PHP Code:

Run-time error '3071': This expression is typed incorrectly, or it is too complex to be evaluated. 
For example, a numeric expression may contain too many complicated elements. 
Try simplifying the expression by assigning parts of the expression to variables. 

I then tried:

Code:
If IsNull(Me.cboManufacturer.Column(0)) Then
MsgBox ("Please select category")
Else
DoCmd.OpenReport "rptItemCategory", acViewPreview
End If

This works if the combo box IS NULL but if i've made a selection, it still gives me the message box as if I've selected nothing....