code: func (c *Client) mustActiveConn() error {
  	c.connsMu.Lock()
  	defer c.connsMu.Unlock()
  
  	for _, c := range c.conns {
  		if !c.IsDead() {
  			return nil
  		}
  	}
  	return errors.Wrap(ErrNoClient, "no active connection found")
  }
docs: // mustActiveConn returns nil if there is an active connection,
  // otherwise ErrNoClient is returned.