Is there a rule that let me avoid having the error message could not force null max?

-- ntp 200 --tr 2.5 --psdwin 0 20 2.5 --ev 1 3 15 -ev 2 3 15  -ev 3 3 15 --ev 4 3 15 ---ev 5 3 15--tnullmin 2 --tnullmax 4.5 ..

Thanks

AS