I have a problem at this question!

using System;
namespace Treehouse.CodeChallenges
{
    public class VocabularyWord
    {
        public string Word { get; private set; }

        public VocabularyWord(string word)
        {
            Word = word;
        }

        public override string ToString()
        {
            return Word;
        }

        public override bool Equals(object obj)
        {
            if(!(obj is VocabularyWord))
            {
                return false;
            }

            var that = obj as VocabularyWord;

            return this.Word.Equals(that.Word);
        }

        public override int GetHashCode(object obj)
        {
            if(!(obj is VocabularyWord))
            {
                return -1; // as an error massage!
            }
            else
            {
                return 7 * Word.GetHashCode();
            }
        }
    }
}