.Net 4.0 System.Web.Security.MembershipProvider含糊不清的参考？

I have recently upgraded my BlogEngine.Net installation to 1.6 and .Net 4.0, however, I did not build BlogEngine.Core when I performed the upgrade. However, when I try to build the BlogEngine.Core project now, the compile fails because it cannot load the symbols for (System.Web.Security.)MembershipProvider. I believe this to be a .Net/C#/user problem rather than a BlogEngine issue.I have recently upgraded my BlogEngine.Net inst