>>You'll have to add check that Thisform.ActiveControl has valid value.
>>
>>That was really fast Sergey, and it works fine. But what about checking
>>for a valid value ?
>
>Under some conditions Thisform.ActiveControl may not point to any control. For example, if form is modeless and looses focus.

IF TYPE("Thisform.ActiveControl") = "O" AND NOT ISNULL(Thisform.ActiveControl)

I placed

Thisform.oPrevControl = IIF(TYPE("Thisform.ActiveControl") = "O" AND NOT ISNULL Thisform.ActiveControl),Thisform.ActiveControl,This.Parent.txtValue)

in the When event of the checkbox.

Thank's for the solution and have a nice weekend.