code: @Override
    public Double zscore(final byte[] key, final byte[] member) {
      checkIsInMultiOrPipeline();
      client.zscore(key, member);
      final String score = client.getBulkReply();
      return (score != null ? new Double(score) : null);
    }
docs: Return the score of the specified element of the sorted set at key. If the specified element
  does not exist in the sorted set, or the key does not exist at all, a special 'nil' value is
  returned.
  <p>
  <b>Time complexity:</b> O(1)
  @param key
  @param member
  @return the score