need Lookup table to return null or zero

I need the lookup fucntion to return a null or a zero when it cannot
find the value required? Currently it is returning the next nearest
value!!

For example:
Col A Col B
07-Jul-05 1
08-Jul-05 4
11-Jul-05 3
12-Jul-05 3

If I lookup for 9-Jul-05 It will return "4" where I need a 0 or null!!

Must be an easy one but I have gone cross eyed!! Chairs