Null value in formula

hi..i have some problem

i have a boolean field on my database. this field has value [true or false AND NULL]

in my formula...
i create like this..

=IF([includeppn]= TRUE();([totalQtyXprice]-[totalDariDiskonPerRow])*(100/110);IF([includeppn]=NULL();[totalQtyXprice]-[totalDariDiskonPerRow];[totalQtyXprice]-[totalDariDiskonPerRow]))

its look like Boolean type cannot read as NULL