The type ‘System.IO.Packaging.Package’ is defined in an assembly that is not referenced. You must add a reference to assembly ‘WindowsBase’

While I was working with the Open Office XML SDK 2.0, after I added the reference to the DocumentFormat.OpenXml assembly and tried to build the application I ran into this error message.

The type ‘System.IO.Packaging.Package’ is defined in an assembly that is not referenced. You must add a reference to assembly ‘WindowsBase, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35′

Turns out, all you need to do to fix it is to just add another reference to WindowsBase assembly that is found in the .NET Tab.