Invalid Contact for 'Create Transaction Contacts Import' API

Hi,
I am using the 'Create Transaction Contacts Import' API to import a contact and getting an error "must provide at least 1 valid contact". I have the batch ID and the parameters in the JSON. Thanks.

{
  "transactionMeta": {
  "batchId": "-------",
  "fields": [
   "string"
  ]
  },
 "contacts": [
  {
   "firstName": "Larry",
   "lastName": "Wang",
   "email": "larry@gmail.com",
   "phone": null,
   "extRef": "13093190",
   "language": null,
   "unsubscribed": true,
   "transactionData" : {}
  }
 ]
}