if (myClass is IComparable<myClass>) not working

So, I was doing a lesson in a book, and as I sometimes do I started playing around with it and trying a few things not specifically spelled out. And, turns out I hit something I don't quite understand. The section was on IComparable<T>. I've worked with interfaces before, so I figured this would work:

using System;
using System.Collections.Generic;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            List<Duck> ducks = new List<Duck>()
            {
                new Duck() { Kind = KindOfDuck.Mallard, Size = 17},
                new Duck() { Kind = KindOfDuck.Muscovy, Size = 18},
                new Duck() { Kind = KindOfDuck.Decoy, Size = 14 },
                new Duck() { Kind = KindOfDuck.Muscovy, Size = 11 },
                new Duck() { Kind = KindOfDuck.Mallard, Size = 14 },
                new Duck() { Kind = KindOfDuck.Decoy, Size = 13 },
            };

           if (ducks is IComparable<Duck>)
                ducks.Sort();
            else
                Console.WriteLine("I can't sort the ducks!");
            
            printDucks(ducks);

            Console.ReadKey();
        }

        private static void printDucks(List<Duck> ducks)
        {
            foreach (Duck duck in ducks)
                Console.WriteLine(duck.Size.ToString() + "-inch " + duck.Kind.ToString());
            Console.WriteLine("End of ducks!");
        }
    }
}

using System;

namespace ConsoleApplication1
{
    class Duck : IComparable<Duck>
    {
        public int Size;
        public KindOfDuck Kind;

        public int CompareTo(Duck other)
        {
            if (this.Size > other.Size)
                return 1;
            else if (this.Size == other.Size)
                return 0;
            else
                return -1;
        }
    }

    enum KindOfDuck
    {
        Mallard,
        Muscovy,
        Decoy,
    }
}

Buy, my console output was:

I can't sort the ducks!
17-inch Mallard
18-inch Muscovy
14-inch Decoy
11-inch Muscovy
14-inch Mallard
13-inch Decoy
End of ducks!

...Whaaaaa? Why isn't if (ducks is IComparable<Duck>) returning true?