java hibernate sequence

@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "username_sequence")
@SequenceGenerator(name = "username_sequence", sequenceName = "partner_user_sequence", allocationSize = 1)
@Column(name = "username", nullable = false, updatable = false)
private Long username;

Caused by: org.postgresql.util.PSQLException: ERROR: null value in column "username" violates not-null constraint

немогу понять что делаю неправильно