categoryが取得できない

test_mapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="person">
  <resultMap id="personMap" type="jp.sample.mybatis.entity.Person">
    <id property="personId" column="person_id" />
    <result property="personId" column="person_id" />
    <result property="name" column="name" />
    <collection property="books" columnPrefix="b_" resultMap="BookMap" />
  </resultMap>
  <resultMap id="BookMap" type="jp.sample.mybatis.entity.Book">
    <id property="bookId" column="book_id" />
    <result property="bookId" column="book_id" />
    <result property="name" column="name" />
    <association property="category" columnPrefix="c_" resultMap="CategoryMap" />
  </resultMap>
  <resultMap id="CategoryMap" type="jp.sample.mybatis.entity.Category">
    <id property="categoryId" column="category_id" />
    <result property="categoryId" column="category_id" />
    <result property="name" column="name" />
  </resultMap>

  <select id="selectByPersonId" resultMap="personMap">
    SELECT
        p.person_id
      , p.name
      , b.book_id     as b_book_id
      , b.name        as b_name
      , c.category_id as c_category_id
      , c.name        as c_name
    FROM
      person p
      INNER JOIN person_book pb USING(person_id)
      INNER JOIN book b USING(book_id)
      INNER JOIN category c USING(category_id)
    WHERE
      p.person_id=#{person_id}
  </select>
</mapper>