Returning null, the .net client dont get null

[HHDirecto . Net]

I have a axis 1.4 web service, My web service return an object, but when the object is null, the .net client don't get object= null, It get object.field1=0, object.field2="" ...

Where is the problem?
Thankss

The webservice response is this:
<?xml version="1.0" encoding="utf-8"?><soapenv:Envelope xmlns:soapenv=" http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi=" http://www.w3.org/2001/XMLSchema-instance"><soapenv:Body><getVariableAnalogicaPorConsignaResponse xmlns="http://interfazweb.serviciosweb.moduloingenieria.xxx.es "><getVariableAnalogicaPorConsignaReturn xsi:nil="true"/></getVariableAnalogicaPorConsignaResponse></soapenv:Body></soapenv:Envelope>

The wsdl file extracts:
<element name="getVariableAnalogicaPorConsigna">
<complexType>
 <sequence>
<element name="idConsigna" type="xsd:long"/>
</sequence>
</complexType>
</element>
<element name="getVariableAnalogicaPorConsignaResponse">
<complexType>
<sequence>
<element name="getVariableAnalogicaPorConsignaReturn" type="impl:VariableAnalogicaIW"/>
</sequence>
</complexType>
</element>
...
<complexType name="VariableAnalogicaIW">
<sequence>
<element name="idFase" type="xsd:long"/>
<element name="idVariableAnalogica" type="xsd:long"/>
<element name="limiteVertidoInferior" type="xsd:double"/>
<element name="limiteVertidoSuperior" type="xsd:double"/>
<element name="nombre" nillable="true" type="xsd:string"/>
<element name="tiempoMuestreo" type="xsd:int"/>
<element name="unidad" nillable="true" type="xsd:string"/>
</sequence>
</complexType>
...
<wsdl:message name="getVariableAnalogicaPorConsignaResponse">
<wsdl:part element="impl:getVariableAnalogicaPorConsignaResponse" name="parameters"/>
</wsdl:message>
<wsdl:message name="getVariableAnalogicaPorConsignaRequest">
<wsdl:part element="impl:getVariableAnalogicaPorConsigna" name="parameters"/>
</wsdl:message>
...
<wsdl:operation name="getVariableAnalogicaPorConsigna">
<wsdl:input message="impl:getVariableAnalogicaPorConsignaRequest" name="getVariableAnalogicaPorConsignaRequest"/>
<wsdl:output message="impl:getVariableAnalogicaPorConsignaResponse" name="getVariableAnalogicaPorConsignaResponse"/>
</wsdl:operation>
...
<wsdl:operation name="getVariableAnalogicaPorConsigna">
<wsdlsoap:operation soapAction=""/>
<wsdl:input name="getVariableAnalogicaPorConsignaRequest">
<wsdlsoap:body use="literal"/>
</wsdl:input>
<wsdl:output name="getVariableAnalogicaPorConsignaResponse">
<wsdlsoap:body use="literal"/>
</wsdl:output>
</wsdl:operation>