Error System.Object

Hello!

I try to run the sample TodoREST but the samarin studio show the error:

C:\TodoREST\TodoREST\TodoREST.cs(15,15): Error CS0012: The type 'System.Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. (CS0012) (TodoREST).

any have idea?

thanks