Excel 2007 : Work Sheet-Reference Relations

Situation:
I am using excel with windows xp. I have set up a worksheet with a reference attached to it. I have two columns. In the first, I type in a code in reference to the reference sheet. For example, when I type in "123" an address will come up. But if I type in a code that doesn't exist on my reference sheet, then a message will appear in the next cell, telling me I put in the incorrect code. But when I delete my incorrect code, the message remains.

Problem:
When my code is deleted, I need for the cell to become empty. My formula for the cell is

=IF(ISNA(VLOOKUP(E3,Ref!$A$2:$D$100,2,FALSE)),"",VLOOKUP(E3,Ref!$A$2:$D$100,2,FALSE))

thank you! it worked perfectly!