Expense Report API Fails with Bad Request when Passing AssignmentId

Hi I am trying to create expense report with REST API. The API used is 

fscmRestApi/resources/11.13.18.05/expenseReports 

The API documentation says that AssignmentId can be passed in payload. However when I pass a valid value in the AssignmentId it fails with Bad Request. When AssignmentId is omitted then the API creates expense report. Can you please help

Body is Below 

{ "PersonId":"300000016617469",

"OrgId":"300000004026131",

"AssignmentId": "300000016617566",

"ReimbursementCurrencyCode": "AUD",

"ExpenseReportTotal":"199.00",

"Purpose":"API Test",

"SubmitReport": "Y"

}