Validation error: "No validator could be found for type: java.lang.Integer"

I am working on a project with Spring why do I keep getting the following error?

javax.validation.UnexpectedTypeException:
No validator could be found for type: java.lang.Integer

Here is my code:

package com.s2rsolutions.model;

import java.util.Date;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;
import javax.validation.constraints.Size;

import org.hibernate.validator.constraints.NotEmpty;

@Entity
@Table(name = "sales")
public class Sales {

    @NotEmpty(message = "The above field must not be blank.")
    @Column(name = "ttl_d_sls_lst_mth", nullable = false)
    private Integer ttl_d_sls_lst_mth;

    @NotEmpty(message = "The above field must not be blank.")
    @Column(name = "ttl_d_sls_6_mth", nullable = false)
    private Integer ttl_d_sls_6_mth;

    @Column(name = "date_added")
    private Date addedDate;

    @Id
    @Column(name = "username")
    private String username;

    // other fields/getters/setters omitted for brevity

}