Why would this code complain about "the arity of the generic type definition"?

I've got a generic type:

class DictionaryComparer<TKey, TValue> : IEqualityComparer<IDictionary<TKey, TValue>>

And a factory method that will (should) create an instance of this class for a given dictionary type.

    private static IEqualityComparer<T> CreateDictionaryComparer<T>()
    {
        Type def = typeof(DictionaryComparer<,>);
        Debug.Assert(typeof(T).IsGenericType);
        Debug.Assert(typeof(T).GetGenericArguments().Length == 2);

        Type t = def.MakeGenericType(typeof(T).GetGenericArguments());

        return (IEqualityComparer<T>)Activator.CreateInstance(t);
    }

Stripping away all of the extraneous stuff - even this code throws the same exception.

private static object CreateDictionaryComparer()
{
    Type def = typeof(DictionaryComparer<,>);

    Type t = def.MakeGenericType(new Type[] { typeof(String), typeof(object) });

    return Activator.CreateInstance(t);
}

The Asserts pass so I know that T is generic and has two generic arguments. The line with MakeGenericType however excepts with:

The number of generic arguments provided doesn't equal the arity of the generic type definition.

Parameter name: instantiation

I've done this sort of thing in the past and for the life of me can't figure out why this isn't working in this case. (plus I had to Google arity).