Problem to Add Reviewers to a work item of RTC from CA side

When we are trying to add Reviewers to a work item of RTC from CA side; there is some error "Error fetching or storing Work Item 88 | Details: Error opening work item. emailAddress is not a valid property for Reviewers". I am sending JSON object in following format:

Approval command structures (JSON).

{"param":{"dueDate":100,"type":"com.ibm.team.workitem.approvalType.review","name":"MyReviewList","approvers":},"cmd":"createApproval"}