Surely that depends on which library versions are defined in your web.config.  

If the web.config points to 3.5 or 4.0 versions of a library then any code targeting a 4.5 specific library function will fail - this is the same case with dnn 6.x not truely supporting asp.net 4.0 functions out of the box because the web.config is targeting 3.5 - even when you are running on an asp.net 4.0 application pool.

You can use straightforward and more secure workaround (Microsoft is working on broad update to address this issue. Contact us if you need immediate hotfix)

Workaround information: Customers can run website in full trust +Process Isolation. More details http://support.microsoft.com/kb/2698981. This approach is recommended approach from Microsoft (from security perspective) as compared to medium trust.