从php soap客户端传递null /空字符串到GUID数据类型时出错

I am using PHP to consume a ASP.NET webservice. This particular method requires a variable of GUID as a parameter. It is defined as Nullable. I am trying to send a null or empty string "" as the parameter. Following which, i get a SoapFault exception: [a:DeserializationFailed] error. Says that The value '' cannot be parsed as the type 'Guid'.'I am using PHP to consume a ASP.NET webservice.