Xamarin.iOS 6.4.4.12 - System.NullReferenceException when trying to cast null value to interface

Following code

    A TestMethod () 
    {
        TestProp = null;
        A testVariable = (A)TestProp;

        return testVariable;
    }

    private A testProp;

    public object TestProp 
    {
        get {
            return this.testProp;
        }
        set {
            this.testProp = (A)value;
        }
    }

    interface A
    {

    }

breaks at line

TestProp = null;

If commented out, then it breaks on

A testVariable = (A)TestProp;

In both cases it fails on

System.NullReferenceException : Object reference not set to an instance of an object
    at Test.Program.set_TestProp (System.Object value) [0x00001] ...

This code is working in Visual Studio 2012 and was working in older versions of Xamarin.

Maybe there is some bug in latest Mono implementation? Do you experience similar problems?