ksoap2: SOAP service receive null argument

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http://stackoverflow.com – I've got a simple SOAP service that receives as String parameter. If i try to invoke it by ksoap2, the server receive a Null argument, and i don't know why. In the following code, i reuse (by initialize it again) variables already used for invoke another service (in the same android method): request = new SoapObject(NAMESPACE, METHOD_NAME2); request.addProperty("lastDate", new String("2013/09/10 14:51:08")); envelope.setOutputSoapObject(request); androidHttpTransport.call(SOAP_ACTION, envelope); envelope.getResponse(); but "lastDate" arg received by server is null. I think it's a ksoap pro (HowTos)