'System.Collections.Generic.IList<object>' does not contain a definition for 'Add' when using 'dynamic' and 'ExpandoObject'

Say that I have a class, Foo, looking something like this:

public class Foo : IFoo
{
    public Foo()
    {
        Bars = new List<dynamic>();
    }
    public IList<dynamic> Bars { get; set; }
}

The interface IFoo looks like:

public interface IFoo
{
    IList<dynamic> Bars { get; set; }
}

Now, when I do the following:

IFoo foo = new Foo();
dynamic a = new System.Dynamic.ExpandoObject();
a.Prop = 10000;
dynamic b = new System.Dynamic.ExpandoObject();
b.Prop = "Some Text";
foo.Bars.Add(a); // Throws an 'System.Collections.Generic.IList<object>' does not contain a definition for 'Add' - exception!!!!!
foo.Bars.Add(b); // Same here!!!!!

What am I doing wrong here?????