Random Quote Generator: Problem with array.property becoming undefined intermittently.

//quotes array of objects
var quotes = [
  {
    quote: "I'm not sure if I was the first man in space or the last dog.",
    source: "-Yuri Gagarin",
    year: "1961",
    tags: "Humor",
  },
  {
    quote: "'We’ll never survive!' 'Nonsense. You’re only saying that because no one ever has.'",
    source: "-William Goldman",
    citation: "The Princess Bride",
    year: "1987",
    tags: "Inspirational",
  },
  {
    quote: "You know, Hobbes, some days even my lucky rocket ship underpants don't help.",
    source: "-Bill Watterson",
    citation: "Calvin and Hobbes",
    tags:"Humor"
  },
  {
    quote: "You people talk about the living and the dead as if they were two mutually exclusive categories. As if you cannot have a river that is also a road, or a song that is also a color.",
    source: "-Neil Gaiman",
    citation:"American Gods",
    year: "2001",
    tags: "Spirituaity"
  },
  {
      quote:"Yet man will never be perfect until he learns to create and destroy; he does know how to destroy, and that is half the battle.",
      source: "-Alexandre Dumas",
      citation: "The Count of Monte Cristo",
      year: "1844",
      tags: "Humanity"
  },
  {
      quote:"Never let your sense of morals prevent you from doing what is right.",
      source: "-Isaac Asimov",
      citation: "Foundation",
      year: "1951",
      tags: "Spirituality"
  },
  {
      quote: "Let's think the unthinkable, let's do the undoable. Let us prepare to grapple with the ineffable itself, and see if we may not eff it after all.",
      source: "-Douglas Adams",
      citation: "Dirk Gently's Holistic Detective Agency",
      year: "1987",
      tags: "Inspirational",
  },
  {
      quote: "A creative man is motivated by the desire to achieve, not by the desire to beat others.",
      source: "-Ayn Rand",
      tags: "Inspirational"
  },
  {
      quote: "I'm not dumb. I just have a command of thoroughly useless information.",
      source: "-Bill Watterson",
      citation: "Calvin and Hobbes",
      tags:"Humor"
  },
  {
      quote:"A bone to the dog is not charity. Charity is the bone shared with the dog, when you are just as hungry as the dog.",
      source:"-Jack London",
      tags:"Humanity"
  },
];

//empty array to hold used quotes
let usedQuotes = [];
//global variable to hold randomly generated quote
var randomQuote;
// function for finding a random quote without repeats
function getRandomQuote() {
   randomQuote = quotes[Math.floor(Math.random()* quotes.length)]
  if(usedQuotes.length === quotes.length){
    usedQuotes = []
  }
  if(usedQuotes.includes(randomQuote) === true){
    getRandomQuote();
  } else {
    usedQuotes.push(randomQuote);
    console.log(randomQuote);
    return randomQuote;
  }
};



// print quotes to page
function printQuote() {
  getRandomQuote();
  let fullQuote = '';
  let outputQuote = document.getElementById('quote-box');
  fullQuote += '<p class="quote">' + randomQuote.quote + '</p>';
  fullQuote += '<p class= "source">' + randomQuote.source;
//check for optional properties
  if(randomQuote.hasOwnProperty('citation')=== true){
    fullQuote += '<span class= "citation">' + ' '+ randomQuote.citation + '</span>';
  }
  if(randomQuote.hasOwnProperty('year')=== true){
    fullQuote += '<span class= "year">' + ' '+ randomQuote.year + '</span>';
  }
  if(randomQuote.hasOwnProperty('tags')=== true){
    fullQuote += '<span class= "tags">' + ' '+ randomQuote.tags + '</span>';
  }
  fullQuote += '</p>';
  outputQuote.innerHTML = fullQuote
}
//Button click event listener
document.getElementById('loadQuote').addEventListener("click", printQuote, false);