Value offer on Filter By Form

[LuboM]

I have a form based on single table. I use Filter by Form on this Form. When
I fill first character to filter field "FirstName", Access offer me in
listbox every unique "firstname" value from table starting with that
character. But if I write first character to field "LastName", Access offer
me only "Is Null" and "Is not Null" values. I need the every unique
"LastName" started with writed character in offer. What have I do?

Thanks