The type or namespace name Caching does not exist in the namespace System.Runtime if CopyLocal is set to False

Hello there, really stuck here someone please let me know what is wrong.  I am using VS2012.

Am having a problem with System.Runtime.Caching.  I referenced the class library, build the solution.  Everything is fine.  But during runtime I will get the following error:

The type or namespace name Caching does not exist in the namespace System.Runtime (are you missing an assembly reference?)

If I change the Copy Local property of System.Runtime.Caching to true, then the issue goes away.  But I cannot keep that setting in production.

I ran the same code on a different machine (different version of VS2012 though) and everything runs fine without changing the Copy Local property.

I have since installed the latest Update 4, and that did not fix it.

Any help is greatly appreciated.

Thank you.