Funny Search Error

I have two fields I am searching on .... address and zipCode





When I have both pieces of data, everything is fine, i get back the expected results, but i am unable able to search on the individual pieces. Here is the SQL I am using. It's placed within a store procedure in Sqlserver that I amreferencing from my VB.net code.... any help??








IF ((NOT @address IS NULL) AND( NOT @zipCode IS NULL))


BEGIN


SELECT OrderNumber FROM Orders WHERE ShpAddr_Address1 = @address AND ShpAddr_ZipCode = @zipCode


END





ELSE IF NOT @address IS NULL


BEGIN


SELECT OrderNumber FROM Orders WHERE ShpAddr_Address1 = @address


END





ELSE IF NOT @zipCode IS NULL


BEGIN


SELECT OrderNumber FROM Orders WHERE ShpAddr_ZipCode = @zipCode


END








any help would be great. thanks