Internal Compiler Error - Lidgren

I recently started using Lidgren Networking for a project, and after coding for a couple hours with no problems Unity started throwing the following error:

  Internal compiler error. See the console log for more information.

output was: Unhandled Exception: System.TypeLoadException: Could not load type 'System.Runtime.Versioning.TargetFrameworkAttribute' from assembly 'Lidgren.Network'.

    at (wrapper managed-to-native) System.MonoCustomAttrs:GetCustomAttributesInternal

(System.Reflection.ICustomAttributeProvider,System.Type,bool)

....... and so on

When I built the dll from Lidgren.Network, I commented out the lines it says to in the wiki, set the target framework to 4.5, and the build configuration to Release x86. I also changed the Api Compatability Level in Unity to .NET 2.0.

I've run out of ideas as to how to make this work again, so I was hoping anyone here could help.

Thanks in advance.