我有一个使用Axis2实现的ws客户端。有些时候有效，但有些时候出现以下错误：
org.apache.axis2.AxisFault: The input stream for an incoming message is null.
at org.apache.axis2.transport.TransportUtils.createSOAPMessage(TransportUtils.java:92)
at org.apache.axis2.transport.TransportUtils.createSOAPMessage(TransportUtils.java:67)
at org.apache.axis2.description.OutInAxisOperationClient.handleResponse(OutInAxisOperation.java:354)
at org.apache.axis2.description.OutInAxisOperationClient.send(OutInAxisOperation.java:421)
at org.apache.axis2.description.OutInAxisOperationClient.executeImpl(OutInAxisOperation.java:229)
at org.apache.axis2.client.OperationClient.execute(OperationClient.java:165)
任何人都可以帮助我吗？