Error: The given id must not be null for GeneratedValue in JPA

My object:

@Entity
@Table(name="user")
public class User {
    @Id
    @Column(name="uid")
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long id;

  //more code
 }

When I POST user JSON without uid, I am getting error as the given id must not be null. Which should not be the case while uid should be generated by the database. Please point what am I missing.

JSON:

{
"email": "john@mail.com",
"name": "John Doe",
"phone": "98-765-4445"
}

Error:

{
"timestamp": 1501058952038,
"status": 500,
"error": "Internal Server Error",
"exception": "org.springframework.dao.InvalidDataAccessApiUsageException",
"message": "The given id must not be null!; nested exception is java.lang.IllegalArgumentException: The given id must not be null!",
"path": "/api/user/"
}