title: Join voice channel (discord.py)
body: <p>When I try to let my bot join my voice channel, I get this error:</p>
  
  <p><code>await client.join_voice_channel(voice_channel)</code> (line that generates the error)</p>
  
  <pre><code>Traceback (most recent call last):
     File "/usr/local/lib/python3.5/site-packages/discord/ext/commands/core.py", line 50, in wrapped
   ret = yield from coro(*args, **kwargs)
   File "bot.py", line 215, in sfx
  vc = await client.join_voice_channel(voice_channel)
  File "/usr/local/lib/python3.5/site-packages/discord/client.py", line 3176, in join_voice_channel
  session_id_future = self.ws.wait_for('VOICE_STATE_UPDATE', session_id_found)
  AttributeError: 'NoneType' object has no attribute 'wait_for'
  </code></pre>
  
  <p>The above exception was the direct cause of the following exception:</p>
  
  <pre><code>Traceback (most recent call last):
   File "/usr/local/lib/python3.5/site-packages/discord/ext/commands/bot.py", line 848, in process_commands
  yield from command.invoke(ctx)
   File "/usr/local/lib/python3.5/site-packages/discord/ext/commands/core.py", line 369, in invoke
  yield from injected(*ctx.args, **ctx.kwargs)
   File "/usr/local/lib/python3.5/site-packages/discord/ext/commands/core.py", line 54, in wrapped
  raise CommandInvokeError(e) from e
  discord.ext.commands.errors.CommandInvokeError: Command raised an exception: AttributeError: 'NoneType' object has no attribute 'wait_for'
  </code></pre>
  
  <p>I'm getting this error with channel name and channel ID</p>
  
  <p>Function:</p>
  
  <pre><code>description = "Bot"
  bot_prefix = "!"
  
  client = discord.Client()
  bot = commands.Bot(description=description, command_prefix=bot_prefix)
  
  @bot.command(pass_context=True)
  async def join(ctx):
     author = ctx.message.author
     voice_channel = author.voice_channel
     vc = await client.join_voice_channel(voice_channel)
  </code></pre>