Conditional Format problems

Hi,
Having fun trying to get Conditional Formatting working on a textbox
control in a continuous form subform (Access 2003). The condition I want
is when the value of the textbox is Null and/or an empty string.

1) Using Access:
I select "Field value is" and "equal to". Not sure what to put in the
Expression1 field. Have tried leaving it empty, inserting a pair of double
quotes (""), a pair of single quotes ('') and the word Null. None of these
work. The condition which I have selected (not enabled) does not happen. I
am sure that there must be a simple solution that I am overlooking.

2) Using VBA:
With my spectacular failure to get 1) working, I turned to VBA. Tried to
apply a FormatCondition to the textbox. As I understand it, the textbox has
a property FormatConditions which refers to a FormatConditions collection
for the textbox. I have tried to use the Add method to add my condition -
eg:

With Me.SubformName!txtBox.FormatConditions
Debug.Print .Count 'prints "1" due to attempt 1) above
.Delete
Debug.Print .Count 'prints "0" - successful delete
.Add acFieldValue, acEqual, Null
'(etc)

This gives me Error 13: Type mismatch (third argument should be variant).
Changing Null to an empty string ("") gives Error 2439: ...wrong number of
arguments.
Changing yet again to some text value (say "X") does not give an error, but
also does not invoke the conditional format when the condition is met.

Obviously I have made an error (probably errors) somewhere.

Any advice for correcting either the Access (1) or VBA (2) version would be
greatly appreciated.

Thanks in advance,
Lyn.
.