Error Saving Workitem with Approval

I'm creating an approval for a workitem but when I save the workitem I
get the error: "Required property must not be null:
internalApprovals.approver"
Does anyone know why? Thanks in advance.


IApprovals approvals = workItem.getApprovals();
IApproval approval = approvals.createApproval(descriptor,
contributorHandle);
approvals.add(approval);

Thanks for the reply. The contributorHandle was fine. I fixed it by
adding this line, but I'm not too sure why it need it...

((Helper) approval).setInternalId(UUID.generate());