Dialog Process throws error

I am executing a Dialog which uses a custom workflow assembly to generate SSRS report and attach the report PDF  to Email. The email activity is passed to the custom workflow as a lookup parameter (EntityReference). When the Dialog is executed then below error is thrown.

Unhandled Exception: Microsoft.Crm.CrmException: The data required to display the next dialog page cannot be found. To resolve this issue, contact the dialog owner or the system administrator.