Value cannot be null. Parameter name: edmEntityObject

When I try to create a Create a Derived Entity with the data given in Tripin Advanced Usages, am getting the following error.

{
"error": {
"code": "",
"message": "Value cannot be null.\r\nParameter name: edmEntityObject"
}
}

Data sent via postman:
header:
{
Content-Type: application/json
}
{
"@odata.type": "Microsoft.OData.Service.Sample.TrippinInMemory.Models.Manager",
"UserName": "mirsking",
"FirstName": "Mirs",
"LastName": "King",
"AddressInfo": [
{
"Address": "Microsoft",
"City": {
"Name": "Shanghai",
"CountryRegion": "China"
}
}],
"BossOffice":{
"Address": "Microsoft",
"City": {
"Name": "Shanghai",
"CountryRegion": "China"
}
},
"Budget": 100
}

Please advise what I am doing wrong.

Review the TripPin Advanced Usages example and compare its derived-entity request with the JSON sent through Postman. Reproduce the request against the referenced service and determine whether the null edmEntityObject error is caused by the payload or the service; done means identifying the required correction or confirming a service defect.