异常为 map evaluated to a null value

Question

<select id="selectRestOfClass" resultType="double" parameterType="map" > select sum(course_num) from crm_order_student_course where
<foreach collection="map" item="item" >
crm_student_id = #{item.crmStudentId}
and subject_id =(select subject_id from crm_student_class where id = #{item.crmStudentClassId})
</foreach>
 and is_deleted=0</select>

Answer 1

map保证不会是空的么？