Import Data replaces NULL with 0

I am using sql server 2008 r2 express. I am getting some problems when i use import data feature to import data from one database to another. I am using SSMS to do it. While using the wizard i choose "Select identity insert" option to copy the identity element property. By using this identity elements are imported properly. But after importing i compared the source database and destination database and found some changes!!. All NULL values in the table are replaced by 0. So how can i avoid it.Is there any option to copy it as it while using import data wizard? Please help