title: Attempt to insert non-property list object when trying to save a custom object in Swift 3
body: <p>I have a simple object which conforms to the <code>NSCoding</code> protocol.</p>
  
  <pre><code>import Foundation
  
  class JobCategory: NSObject, NSCoding {
      var id: Int
      var name: String
      var URLString: String
  
      init(id: Int, name: String, URLString: String) {
          self.id = id
          self.name = name
          self.URLString = URLString
      }
  
      // MARK: - NSCoding
      required init(coder aDecoder: NSCoder) {
          id = aDecoder.decodeObject(forKey: "id") as? Int ?? aDecoder.decodeInteger(forKey: "id")
          name = aDecoder.decodeObject(forKey: "name") as! String
          URLString = aDecoder.decodeObject(forKey: "URLString") as! String
      }
  
      func encode(with aCoder: NSCoder) {
          aCoder.encode(id, forKey: "id")
          aCoder.encode(name, forKey: "name")
          aCoder.encode(URLString, forKey: "URLString")
      }
  }
  </code></pre>
  
  <p>I'm trying to save an instance of it in <code>UserDefaults</code> but it keeps failing with the following error.</p>
  
  <blockquote>
    <p>Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attempt to insert non-property list object  for key jobCategory'</p>
  </blockquote>
  
  <p>This is the code where I'm saving in <code>UserDefaults</code>.</p>
  
  <pre><code>enum UserDefaultsKeys: String {
      case jobCategory
  }
  
  class ViewController: UIViewController {
  
      @IBAction func didTapSaveButton(_ sender: UIButton) {
          let category = JobCategory(id: 1, name: "Test Category", URLString: "http://www.example-job.com")
  
          let userDefaults = UserDefaults.standard
          userDefaults.set(category, forKey: UserDefaultsKeys.jobCategory.rawValue)
          userDefaults.synchronize()
      }
  }
  </code></pre>
  
  <p>I replaced the enum value to key with a normal string but the same error still occurs. Any idea what's causing this?</p>