sharepoint - GetListItems is throwing exception for a combination of viewFields -

i calling getlistitems webservice data sharepoint 2010 server. webservice working fine when not provide value viewfield argument or when call webservice of individual fields in list.

but combinations of fields webservice throwing exception. following content of exception.

exception of type 'microsoft.sharepoint.soapserver.soapserverexception' thrown.

server stack trace: @ system.servicemodel.channels.servicechannel.handlereply(proxyoperationruntime operation, proxyrpc& rpc) @ system.servicemodel.channels.servicechannel.call(string action, boolean oneway, proxyoperationruntime operation, object[] ins, object[] outs, timespan timeout) @ system.servicemodel.channels.servicechannel.call(string action, boolean oneway, proxyoperationruntime operation, object[] ins, object[] outs) @ system.servicemodel.channels.servicechannelproxy.invokeservice(imethodcallmessage methodcall, proxyoperationruntime operation) @ system.servicemodel.channels.servicechannelproxy.invoke(imessage message)

exception rethrown @ [0]:

any idea reason exception. surprising thing individual field values not creating problem combination is.

i have been working sharepoint webservices last 6 year not seen such issue.