Workflow - Error

Hi everyone... I have an error with my workflow. I am using Aras 11.0 SP9. When i create an instance of ItemType, It flow over my custom workflow. In my workflow i defined as creator submit to the reviewer. After review process, if he reject means it will again come to creator. Creator will resubmit. Error coming while i am resubmitting... It shows as "Conversion from type 'DBNull' to type 'String' is not valid." i can not resubmit to reviewer.   How to resolve this issue? Regards, Sathishkumar C.