random word generator that re-rolls if it generate a specific word

I'm trying to make a random word generator that re-rolls if the generated word is "empty slot" but I get an error "exception unhandled" if the generated word is "empty slot"error my code:

    public void rdwrd()
        {
                    Random r = new Random();

    string[] words = { word1, word2, word3, word4, word5, word6, word7, word8, word9, word10, word11, word12, word13, word14, word15, word16, word17, word18, word19, word20, word21, word22 };
        if((words[r.Next(0, words.Length)]) == "empty slot")
        {
            rdwrd();
        }
        else
    Console.WriteLine(words[r.Next(0, words.Length)]);

        answerbox.Text = (words[r.Next(0, words.Length)]);
    }

i know this might be stupid but new to coding, i would really appreciate if anybody could help