expects parameter 1 to be mysqli_result, boolean given in hatası

Warning: mysqli_fetch_array() expects parameter 1 to be mysqli_result, boolean given in /home/sitem/public_html/_lib/class.Db.php on line 64

while ($line = mysqli_fetch_array($result, MYSQL_ASSOC))

<?php
class ConnectException extends Exception {}
class QueryException extends Exception {}

class Db extends mysqli
{
    function __construct($host, $username, $passwd, $dbname, $port)
    {
        parent::__construct($host, $username, $passwd, $dbname, $port);

        /* Throw an error if the connection fails */
        if(mysqli_connect_error() && ENVIRONMENT == 'dev')
        {
            throw new ConnectException(mysqli_connect_error(), mysqli_connect_errno());
        }
    }
    
    public function query($query)
    {
        $result = parent::query($query);
      if(mysqli_error($this) && ENVIRONMENT == 'dev')
        {
            throw new QueryException(mysqli_error($this), mysqli_errno($this));
        }
        return $result;
    }
    
    public function q($query)
    {
        $result = parent::query($query);
        
        if (mysqli_error($this))
        {
            throw new Exception(mysqli_error($this), mysqli_errno($this));
        }
        
        $array_result = array();
        while ($line = mysqli_fetch_array($result, MYSQL_ASSOC))
        {
            $NewLine = array();
            foreach($line as $key=>$val)
                $NewLine[$key] = stripslashes($val);
            $array_result[] = $NewLine;
        }
        unset($result, $line);
        return $array_result;
    }
    
    public function QueryArray($query)
    {
        $result = parent::query($query);
        $array_result = array();
        while ($line = mysqli_fetch_array($result, MYSQL_ASSOC))
        {
            $array_result[] = $line;
        }
        unset($result, $line);
        return $array_result;
    }
    
    public function QueryRow($query)
    {
        $result = parent::query($query);
        $line = mysqli_fetch_array($result, MYSQL_ASSOC);
        if (empty($line))
            return false;
        $NewLine = array();
        foreach($line as $key=>$val)
            $NewLine[$key] = stripslashes($val);
        return $NewLine;
    }

    // Runs a query and returns result as a single variable
    public function QueryItem($query)
    {
        $result = parent::query($query);
        if (!$result)
        {
            return false;
        }
        $line = mysqli_fetch_array($result, MYSQL_NUM);
        if ($line)
        {
            return stripslashes($line[0]);
        }
        return false;
    }
    
    public function Execute($query)
    {
        $result = parent::query($query);
        if(mysqli_error($this))
        {
            throw new Exception(mysqli_error($this), mysqli_errno($this));
            return false;
        }
        else
        {
            return true;
        }
    }
    
    public function GetServerInfo()
    {
        return $this->server_info;
    }
    
    public function ExecuteMultiple($query)
    {
        $result = parent::multi_query($query);
        if(mysqli_error($this) && ENVIRONMENT == 'dev')
        {
            throw new QueryException(mysqli_connect_error(), mysqli_connect_errno());
            return false;
        }
        else
        {
            return true;
        }
    }
}
?>