I create function to read as:

=IF(D6=40,"Pass",IF(D6=1,"Fail",IF(D6=0,"")))

my question, when I enter mark 40 and above it appear 'Pass" - ok
when I enter below 40 it appear 'Fail' - ok
but if I enter - (still show 'Pass')

how to overcome like when I enter 0 - become blank