Why do 'abstract' methods with 'in' or 'out' contracts require a body?

[Trey Brisbane via Digitalmars-d-learn]

Hey all,

I have a class method defined like so:
abstract class MyClass {
public:
        @property
        abstract SomeClassType getField() pure nothrow
        out(result) {
                assert(result !is null, "Error: getField() returned null.");
        }
}

As you can see, this method is abstract, as well as in an 
abstract class. Why, then, do I get the following error when 
compiling it?

"Error: function module.MyClass.getField in and out contracts 
require function body"

Is this a compiler bug, or is there a reason for this?

Thanks for your time!