If you use a checkbox with a nullable field,
and bind this field to a checkbox,
with CTRL+0 or set .Value to NULL not fill the field with NULL.

PUBLIC oform1

oform1=NEWOBJECT("form1")
oform1.Show
RETURN

DEFINE CLASS form1 AS form
	autocenter = .T.
	DataSession = 2
	Top = 0
	Left = 0
	Height = 250
	Width = 314
	DoCreate = .T.
	Caption = "Form1"
	AllowOutput = .F.
	Name = "Form1"


	ADD OBJECT check1 AS checkbox WITH ;
		Top = 51, ;
		Left = 73, ;
		Height = 17, ;
		Width = 60, ;
		Caption = "Check1", ;
		ControlSource = "checkbinded", ;
		Name = "Check1"


	PROCEDURE Load
		CREATE CURSOR mycursor (checkbinded L NULL DEFAULT NULL )
		APPEND BLANK
		clear
	ENDPROC

	PROCEDURE check1.InteractiveChange
		? PROGRAM(),this.value
		this.Value = NULL
	ENDPROC

	PROCEDURE check1.Valid
		? PROGRAM(),this.value,EVALUATE(m.this.ControlSource)
	ENDPROC

	PROCEDURE check1.LostFocus
		? PROGRAM(),this.value,EVALUATE(m.this.ControlSource)
	ENDPROC

ENDDEFINE