Benjamin,

NULL and empty are two distinct different values in VFP. When the user blanks out a field of its data, they create an empty field. The keystroke CTRL-0 (zero number) enters a value of NULL into the field.

You can check the value of the control in the valid and if empty replace it with NULL;

* control valid
IF EMPTY(This.Value)
   This.Value = .NULL.
ENDIF