Cast some variables as strings to prevent errors

lance_santuyo_reading/bookshelf/models.py

@@ -10,7 +10,7 @@ class Author(models.Model):
     bio = models.TextField(blank=True, null=True)
 
     def __str__(self):
-        return self.first_name + ' ' + self.last_name
+        return str(self.first_name) + " " + str(self.last_name)
 
     def get_absolute_url(self):
         return reverse('bookshelf:author-item', kwargs={'pk': self.pk})