When lookign up a range why does the =LOOKUP(MONTH) give values even if the
cell is blank

If I have
no date in A1 but have a formula in C1
(=LOOKUP(MONTH(RC[-2]),{0,1,2,3,4,5,6,7,8,9,10,11,12},{"
","JAN","FEB","MAR","APR","MAY","JUN","JUL","AUG", "SEP","OCT","NOV","DEC"})

it give me a value of Jan is there a way around this?