penyebab error pada query update menggunakan prepare statement ?

mohon bantuanya gan,

output saat di jalankan di browser : Warning: mysqli_stmt::bind_param(): Number of elements in type definition string doesn't match number of bind variables in /home/ivan/Project/PHP/Experiment/phptest/oop/blog/classes/DB.php on line 64

code saya :

    public function update_data($table,$id,$fields=[]){

        //for key
        $x = 1;
        foreach ($fields as $key => $value) {
            $set .= "{$key} = ?";
            if($x < count($fields)){
                $set .= ", ";
            }
            $x++;
        }
        $query = "UPDATE $table SET $set WHERE id = $id";

        //for value
        $y = array_values($fields);
        $val = implode(',',$fields);
        $result = $this->conn->prepare($query);
        $result->bind_param("ssss",$val);
        $result->execute();


    }

nilai variable $query : UPDATE tb_blog SET title = ?, dateBlog = ?, content = ?, tag = ? WHERE id = 2

nilai variable $val : title 2 coba,2018-06-20,lallalaaallala lallalaaallala lallalaaallala lallalaaallala lallalaaallala,semester 1

kira2 kesalahanya dimana ya ? terimakasish