Establish Sessions Error REST API

Summary

Cannot set contact email, name etc...

Content

I am trying to create a chat using REST API. When initiating a chat through establishSession endpoint, I have give details as shown  

{
 "sessionInformation": {
 "emailAddress" : "john.smith@vision.com", 
 "firstName" : "John",
 "lastName": "Smith", 
 "question" : "Refunds Issues"
 }
}

But it is showing an error like given below. Is there any solution.

"title": "An attempt to write a resource failed",

    "status": 400,

    "detail": "A problem setting a property was encountered: not an object; EstablishSession.SessionInformation",

"o:errorCode": "OSC-CREST-00006"