CATIA VBScript не понимает конструкцию is Not Nothing 
 зато понимает is Nothing 
 т.е. :  function fun1(a as Object) as Object 
 set fun1 = Nothing 
 end function 
 function fun2(a as Object) as Object 
 set fun2 = 2 
 end function 
 '... main ... 
 Dim i as Object 
 set i = fun1(1) 
 if i is Nothing then 
 MsgBox "i is Nothing" 
 else 
 MSgBOx "i is not Nothing" 
 end if 
 set i = fun2(2) 
 if i is Nothing then 
 MsgBox "i is Nothing" 
 else 
 MSgBOx "i is not Nothing" 
 end if 
 ... 
 выведет: 
 i is Nothing 
 i is not Nothing 
 т.е. норм. отработает 
 if Not( i is Nothing) также не работает 
 if (i is Nothing)=true или false также не работает 
 PS: а Not вообще зачем тогда?