Setup a method with argument of interface type doesn't work when a not null instance is passed to that method

[jsayari]

I am trying to setup a method which accepts a parameter of an interface type. To make it clear suppose that I am mocking the following IFoo interface and its DoSomething method accept parameter of IBar interface:

public interface IFoo
{
     int DoSomething(IBar bar);
}
public class Bar : IBar
{
}

Then setting up the IFoo mocking object as:

var moqObject = new Moq<IFoo>();
moqObject.Setup(x => x.DoSomething(It.IsAny<IBar>())).Returns(() => 10);

When calling DoSomething method of the moq object with null argument it works correctly and returns 10 as it was set up but when call it with a new instance of Bar it doesn't go into the Returns part.

moqObject.Object.DoSomething(null);//this returns 10 and works as expected
moqObject.Object.DoSomething(new Bar()); //this doesn't enter the Rerturns part of the moqObject

But if I cast the Bar object into IBar it works correctly

moqObject.Object.DoSomething((IBar)new Bar());//this returns 10 as well

Anything wrong with my mocking object setup? or this behavior is by design?