title: discord.ext.commands.errors.CommandInvokeError: Command raised an exception: AttributeError: 'NoneType' object has no attribute 'mention'
body: <p><strong>My code:</strong></p>
  <pre class="lang-py prettyprint-override"><code>
  @commands.command(pass_context=True)
  @commands.guild_only()
  async def owner_mention(self, ctx):
      await ctx.send(ctx.guild.owner.mention)
  </code></pre>
  <p><strong>Error:</strong></p>
  <blockquote>
  <p>discord.ext.commands.errors.CommandInvokeError: Command raised an
  exception: AttributeError: 'NoneType' object has no attribute
  'mention'</p>
  </blockquote>