WORKFLOW_REJECT : Null pointer is dereferenced

Using WCC 11.1.1.8.0  On calling WORKFLOW_REJECT service, the following error is thrown:

An error has occurred. The stack trace below shows more information.

Event generated by user 'test1' at host 'localhost:16200'. Unable to send mail to contributors. Unable to execute service method 'prepareMailForStepUsers'. Null pointer is dereferenced.

!csUserEventMessage,test1,localhost:16200!$!csWfContributorMailError!csUnableToExecMethod,prepareMailForStepUsers!syNullPointerException

intradoc.common.ServiceException: !csWfContributorMailError!csUnableToExecMethod,prepareMailForStepUsers

*ScriptStack WORKFLOW_REJECT

3:prepareMailForStepUsers,dID=1234567,dDocName=D_1234567

        at intradoc.server.ServiceRequestImplementor.buildServiceException(ServiceRequestImplementor.java:2176)

        at intradoc.server.Service.buildServiceException(Service.java:2402)

        at intradoc.server.Service.createServiceExceptionEx(Service.java:2396)

        at intradoc.server.Service.createServiceException(Service.java:2391)

        at intradoc.server.ServiceHandler.executeAction(ServiceHandler.java:102)