Correct NHibernate mapping for stored procedure?

UPDATE: this is now allowed, see answer below.

I'm having trouble trying to work out the correct way to write the NHibernate mapping file (.hbm.xml) for the MSSQL stored procedure.

A stored procedure takes two parameters and returns a single row row result set containing multiple columns that have integer values.

My mapping file is as follows:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   assembly="TestApp.DataAccess"
                   namespace="TestApp.DataAccess.Models">

  <class name="MonthlyInstructionCount" lazy="true">
    <id name="Id" column="Month">
      <generator class="native" />
    </id>

    <property name="Month" />
    <property name="MapRequests" />
    <property name="Instructions" />
    <property name="DrainsLookSee" />
    <property name="DrainsFullCctv" />
    <property name="Soils" />
    <property name="Roots"/>
    <property name="Arb" />

    <loader query-ref="MI_MonthlyInstructionCount"/>
  </class>

  <sql-query name="MI_MonthlyInstructionCount">
    <return class ="MonthlyInstructionCount">
      <return-property name="Month" column="Month" />
      <return-property name="MapRequests" column="MapRequests" />
      <return-property name="Instructions" column="Instructions" />
      <return-property name="DrainsLookSee" column="DrainsLookSee" />
      <return-property name="DrainsFullCctv" column="DrainsFullCctv" />
      <return-property name="Soils" column="Soils" />
      <return-property name="Roots" column="Roots" />
      <return-property name="Arb" column="Arb" />
    </return>
    exec dbo.MI_MonthlyInstructionCount :StartDate :NumberOfMonths
  </sql-query>

</hibernate-mapping>

I also tried the following, as suggested by someone on the forum for a similar problem that another person had to do with stored procedures ...

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   assembly="TestApp.DataAccess"
                   namespace="TestApp.DataAccess.Models">

  <sql-query name="MI_MonthlyInstructionCount">
    <return class ="MonthlyInstructionCount" />
    exec dbo.MI_MonthlyInstructionCount :StartDate :NumberOfMonths
  </sql-query>

</hibernate-mapping>

None of them work ... I just get the following error:

The type initializer for 'TestApp.DataAccess.Sql.NHibernateHelper' threw an exception.

{ " : {MI_MonthlyInstructionCount}" }

, ( , ...), , , MSSQL.

! !