a strange param of a WS

I am writing a client of this service:
http://ws.keyfortravel.com/Air/airfares.asmx?WSDL

This .Net site provides a test page of the operation:
http://ws.keyfortravel.com/Air/airfares.asmx?op=GetFares

totally confused by the first parameter "xml", type String.
Using this test page, I don't have to input anything and it works fine. But in my code, I tried new String() and null, axis throws exception.

Anyone get clue on this? Many thanks

AxisFault
faultCode: {http://schemas.xmlsoap.org/soap/envelope/}Server.userException
faultSubcode:
faultString: java.lang.NumberFormatException: Invalid positiveInteger: 0
faultActor:
faultNode:
faultDetail:
{http://xml.apache.org/axis/}stackTrace:java.lang.NumberFormatException: Invalid positiveInteger: 0
at org.apache.axis.encoding.ser.SimpleDeserializer.onEndElement(SimpleDeserializer.java:180)
at org.apache.axis.encoding.DeserializerImpl.endElement(DeserializerImpl.java:506)
at org.apache.axis.encoding.DeserializationContextImpl.endElement(DeserializationContextImpl.java:1060)
at org.apache.axis.message.SAX2EventRecorder.replay(SAX2EventRecorder.java:165)
at org.apache.axis.message.MessageElement.publishToHandler(MessageElement.java:838)
at org.apache.axis.message.RPCElement.deserialize(RPCElement.java:198)
at org.apache.axis.message.RPCElement.getParams(RPCElement.java:323)
at org.apache.axis.client.Call.invoke(Call.java:2418)
at org.apache.axis.client.Call.invoke(Call.java:2317)
at org.apache.axis.client.Call.invoke(Call.java:1774)
at pt.viatecla.k4t.k4t.AirFares.K4TAirFaresWSSoapStub.getFares(K4TAirFaresWSSoapStub.java:362)
at pt.viatecla.k4t.k4t.AirFares.K4TAirFaresWSTestCase.test1K4TAirFaresWSSoapGetFares(K4TAirFaresWSTestCase.java:34)

Hello "yaoheng"-

Welcome to JavaRanch.

On your way in you may have missed that JavaRanch has a policy on display names, and yours does not comply with it - please adjust it accordingly, which you can do right here. Thanks for your prompt attention to this matter.

Enyoy your time here.

seems the date params cause the exception, but still don't know how to deal with it.

i send the data format like 01/10, the client works and web services return a error message said the invalid data. But if i send 2006/02/01, axis throws exceptions as it in the previous post.