Crystal reports 8.5 and RDC 6.0

Initially I developed my project using VB 6.o with RDC 6.0 as reporting tools and Oracle as back-end. The system was deployed and working fine. I upgraded the project with the crystal report 8.5 recently and problems start. After deployed the new upgraded system, I cann’t view the report anymore. I always got the ‘Invalid use of null’ runtime error. I’m sure that the database is working fine. And I didn’t change anything in my reports. The purpose that I upgrade the system is because the RDC 6.0 lack of some functionalities required and the report loading speed is very slow. I’m thinking a way to resolve this problem by changing from RDC 6.0 to crystal reports 8.5. But i’m facing this problem now. Anyone could help me? Thanks 🙂