Key comparisons for Linq GroupBy using Default EqualityComparer

I'm trying to do a Linq GroupBy on some objects using an explicit key type. I'm not passing an IEqualityComparer to the GroupBy, so according to the docs:

The default equality comparer Default is used to compare keys.

It explains the EqualityComparer<T>.Default property like this:

The Default property checks whether type T implements the System.IEquatable<T> generic interface and if so returns an EqualityComparer<T> that uses that implementation.

In the code below, I'm grouping an array of Fred objects. They have a key type called FredKey, which implements IEquatable<FredKey>.

That should be enough to make the grouping work, but the grouping is not working. In the last line below I should have 2 groups, but I don't, I just have 3 groups containing the 3 input items.

Why is the grouping not working?

class Fred
{
    public string A;
    public string B;
    public FredKey Key
    {
        get { return new FredKey() { A = this.A }; }
    }
}

class FredKey : IEquatable<FredKey>
{
    public string A;
    public bool Equals(FredKey other)
    {
        return A == other.A;
    }
}

class Program
{
    static void Main(string[] args)
    {
        var f = new Fred[]
        {
            new Fred {A = "hello", B = "frog"},
            new Fred {A = "jim", B = "jog"},
            new Fred {A = "hello", B = "bog"},
        };

        var groups = f.GroupBy(x => x.Key);
        Debug.Assert(groups.Count() == 2);  // <--- fails
    }
}