Type not defined error

I am getting the following error when trying to open an asp.net page that we are building.  The error does not occur on our server but it does on an identical web site on a customer server.  It works on all workstations in our domain.  I suspect it is some issue at the asp.net web server, but I cannot tell.  Both server, ours and customers have the exact same web pages and the exact same files in the bin folder.
error BC30002: Type 'Warning' is not defined.
 Stack Trace:    at System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath)    at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile)    at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPr ...