Why is a RuntimeBinderException thrown when an inherited method is called on the interface type passing a dynamic object?

Question

Can you please explain the behavior for this code:

namespace DynamicTesting
{
    public interface IDynamicTargetBase
    {
        string Hello(int a);
    }

    public interface IDynamicTarget : IDynamicTargetBase
    {
    }

    public class DynamicTarget : IDynamicTarget
    {
        public string Hello(int a)
        {
            return "Hello!";
        }
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            dynamic a = 123;

            IDynamicTargetBase obj1 = new DynamicTarget();
            obj1.Hello(a);  // This works just fine

            IDynamicTarget obj2 = new DynamicTarget();
            obj2.Hello(a); // RuntimeBinderException "No overload for method 'Hello' takes '1' arguments"
        }
    }
}

The result is:

[Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: No overload for method 'Hello' takes '1' arguments] at CallSite.Target(Closure , CallSite , IDynamicTarget , Object ) at System.Dynamic.UpdateDelegates.UpdateAndExecuteVoid2[T0,T1](CallSite site, T0 arg0, T1 arg1) at DynamicTesting.Program.Main(String[] args) :line 30

If I use int instead of dynamic then it's working fine. I cannot get why it's not working. Do you have any ideas?

Try it on your own: https://dotnetfiddle.net/55ZMAG