Help error cannot add or update a child row: a foreign key constraint fails

Hallo saya mau bertanya, saya punya 2 tabel, tabel pertama bernama "jurusan" yang isinya: id_jurusan(pk) , nama_jurusan. Terus tabel kedua bernama "kelas" yang isinya: id_kelas(PK), nama_kelas, id_jurusan(FK), nah saat ingin input data ke table melalui form "kelasadd.php" terjadi error seperti ini "Cannot add or update a child row: a foreign key constraint fails (`sistem`.`siswa`, CONSTRAINT `siswa_ibfk_1` FOREIGN KEY (`jurusan`) REFERENCES `jurusan` (`id_jurusan`))"

Kira-kira solusinya gimana ya? terimakasih sebelumnya.

ini source codenya

code pada formnya

<?php include "head.php";
include "sidebar.php"; ?>

<div class="col-md-3 p-5 pt- ">

    <form action="kelasproses.php" method="post">

        <div class="form-group">
            <label for="nama">Nama Kelas</label>
            <input type="text" class="form-control" id="nama" name="nama" required>
        </div>

        <div class="form-group">
            <select name="jurusan" class="custom-select  mb-2">
                <?php include "../conf/config.php";
                $sql_kelas = mysqli_query($con, "SELECT * FROM jurusan") or die(mysqli_error($con));
                while ($jurusan = mysqli_fetch_assoc($sql_kelas)) {
                    echo '<option value="' . $sql_jurusan['id_jurusan'] . '">' .
                        $jurusan['nama_jurusan'] . '</option>';
                }
                ?>
            </select>
        </div>

        <div class="form-group row">
            <div class="col-sm-10">
                <button type="submit" name="add" class="btn btn-primary">Simpan</button>
            </div>
        </div>
    </form>

</div>

<?php include "footer.php" ?>

code prosesnya

<?php
require_once "../conf/config.php";

if (isset($_POST['add'])) {

    $nama = trim(mysqli_real_escape_string($con, $_POST['nama']));
    $jurusan = trim(mysqli_real_escape_string($con, $_POST['jurusan']));


    mysqli_query($con, "INSERT INTO kelas ( nama_kelas, id_jurusan) VALUES ('$nama', '$jurusan')") or die(mysqli_error($con));
    echo "<script>window.location='kelasdata.php';</script>";
}