Vb.net problem with web service wsdl SOAP return HTTP 500

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VB

SOAP

WSDL

Actually I have the following webmethod:

_
Public Function deliveryNotification(transId As String, messageId As String, [from] As String, [to] As String, status As Integer, tariffId As System.Nullable(Of Integer), price As System.Nullable(Of Double), errorCode As System.Nullable(Of Integer), errorMessage As String) As <XmlElement("genericResponse")> type.ws.cdc.tecnotree.com.CDCCommonType.GenericResponseType

the webservice work fine but when my cliente send me tag "errorCode" empty the web service return HTTP 500 (Internal Error)
There is an error in XML Document (23,41).

This is the XML body TAG empty of my client:

<errorCode></errorCode>

My client work with with Java. I have made a client in Net and work fine but the XML body tag is the following:

<errorCode xsi:nil="true"/>

Somebody help me please.

Thanks,

What I have tried:

I need the webservice working with the tag empty as supplied by the client