title: Can't get discord.js to play a song with ytdl-core
body: <p>so, i tried doing a ytdl thing with discord.js. i get an error saying</p>
  
  <blockquote>
    <p>(node:6796) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): TypeError: Parameter "url" must be a string, not undefined".</p>
  </blockquote>
  
  <p>this is where the bot is supposed to play something. ((yes ytdl is defined))</p>
  
  <pre><code>const dispatcher = connection.playStream(ytdl(args[1]))
  .on("end", () =&gt; { 
      console.log("song end");
  })
  .on("error", () =&gt; {
      console.error(error); 
  });
  </code></pre>