Erro na bag object reference not set to an instance of an object

[William Rodrigues]

Essa bag está vazia, ao invés de retornar null está retornando com erro.

<bag name="ServicoMensagemGestor" cascade="all-delete-orphan" inverse="true">

      <key>

        <column name="SMR_CDCLIENTE"/>

        <column name="SMR_CDGESTOR"/>

        <column name="SMR_CDMANDATO"/>

      </key>

      <one-to-many class="ServicoMensagemGestor" />

    </bag>

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="APSA.Condominio.Domain" namespace="APSA.Condominio.Domain" >

  <class name="ServicoMensagemGestor" table="SERVICOMSGGESTOR_SMR">

    <id name="Codigo" column="SMR_CDSERVICOMSGGESTOR">

      <generator class="sequence">

        <param name="sequence">SEQ1_SERVICOGESTOR_SMR</param>

      </generator>

    </id>

    <property name="TipoOperacao" column="SMR_TIPOOPERACAO"/>

    <property name="DataOperacao" column="SMR_DTPROCESSO"/>

    <property name="Usuario" column="SMR_USUARIO"/>

    <many-to-one name="ServicoMensagem" class="ServicoMensagem" column="SMR_CDSERVICOMENSAGEM"/>

    <many-to-one name="AgendamentoEnvioMensagemSMSEmail" class="AgendamentoEnvioMensagemSMSEmail" column="SMR_CDAGENDA"/>

    <many-to-one name="MandatoGestor" class="MandatoGestor">

      <column name="SMR_CDCLIENTE"/>

      <column name="SMR_CDGESTOR"/>

      <column name="SMR_CDMANDATO"/>

    </many-to-one>

  </class>

</hibernate-mapping>

Stack Trace:    em NHibernate.Collection.PersistentBag.get_Count()

   em System.Linq.Buffer`1..ctor(IEnumerable`1 source)

   em System.Linq.OrderedEnumerable`1.<GetEnumerator>d__0.MoveNext()

   em System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)

   em System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)