>>>Might you have the variable "NULL" set to an empty string somewhere?
>>>
>>>good luck
>>>
>>IF .lAddblank
>> =MESSAGEBOX(.name + " null? " + TRANSFORM(ISNULL(this.aRowSource[1,2])))
>> endif
>>
>>Shows .t. Why would it change to blank?
>
>Maybe you're looking at the wrong one.

No, it was the one. But I just found that changing BoundTo property of the combobox solves the problem. With boundto = .f. it sets the value to blank instead of NULL. With this property set to true, it shows NULL, as I need.