Can't use DBNull in .NET Standard 1.4?

I am creating a class library that will be used in a WPF project and a .NET Core project.

For the following code:

    public class MyClass
    {
        private void MyFunction(object o)
        {
            if (o == DBNull)
            {
                //ommitted
            }
        }
    }

I am getting the following error:

The name 'DBNull' does not exist in the current context

This is a .NET Standard Class Library Project created in Visual Studio 2017.

Both my .NET Core project and WPF Project allow the use of DBNull.