无法在Visual Studio 2010中将.asmx文件添加为web引用

当我尝试向我的VS2010项目添加web引用时,我遇到了以下错误(服务运行正常,实际上VS2010在查找系统上托管的服务时会自己检测到它)

The document at the url http://localhost/_vti_bin/PPS/PPSAuthoringService.asmx 
was not recognized as a known document type.
The error message from each known type may help you fix the problem:
- Report from 'XML Schema' is 'The document format is not recognized
 (the content type is 'text/html; char