Null value in form not trapped by beforeupdate event

[SUZYQ]

I have a form in Access 2003 linked to a SQL Server 2005 table. When
I clear the value in a textbox (bound field is varchar and is
required), I want the before update event to run to tell the user the
value cannot be null.

When I press the tab button to move to the next field after clearing
the texbox, the before update event is not triggering and instead I'm
getting the following error:

You tried to assign the Null value to a variable that is not a Variant
data type. (Error 3162)

How can I prevent nulls before and this error from triggering?

Thanks!