mybatis中传入String类型参数异常

在使用mybatis时，写了一条sql语句，只有一个String类型的参数， 示例代码

<select id="getApplyNum" parameterType="java.lang.String" resultType="java.util.Map">
         SELECT
            t.activity_id AS "activityId",
            COUNT(1) AS "count"
        FROM
            t_user t
        WHERE
            t.activity_id IN (${ids})
        GROUP BY
            t.activity_id
 </select>

Mybatis查询sql传入一个字符串传参数，报There is no getter for property named 'ids' in 'class java.lang.String'。

后来改成如下写法，无论参数名是啥，都要改成"_parameter"。问题解决。

 <select id="getApplyNum" parameterType="java.lang.String" resultType="java.util.Map">
         SELECT
            t.activity_id AS "activityId",
            COUNT(1) AS "count"
        FROM
            t_user t
        WHERE
            t.activity_id IN (${_parameter})
        GROUP BY
            t.activity_id
</select>