title: predis: ZADD with NX keeps throwing error 'Predis\ServerException'
body: <p>I am having an issue when I try to add "NX" to the zadd command on predis.  The redis docs say that ZADD should support NX, but no matter how I set up the predis command, I can not get it working.  Does anyone have any experience with this issue?</p>
  
  <p>Here are the commands I have tried:</p>
  
  <pre><code>$redis-&gt;zadd($key, "NX", 1, $id);
  
  $redis-&gt;executeRaw([ 'ZADD', $key, "NX", 1, $id ]);
  </code></pre>
  
  <p>Here is the error that keeps getting thrown:</p>
  
  <pre><code>ERROR: exception 'Predis\ServerException' with message 'ERR syntax error'
  </code></pre>
  
  <p>Looking at the redis-cli monitor, I see the command execute when using the ZADD command, but the executeRaw command does nothing.</p>
  
  <p>Any help would be greatly appreciated!</p>