如何消除致命错误＆＃34;不能为空＆＃34;？

Question

我试图将多维数组转换为数据库中的单个表，但收到以下错误＆＃34;致命错误：未捕获异常＆＃39; mysqli_sql_exception＆＃39;与消息＆＃39;列＆＃39;日＆＃39;不能为空＆＃39;在C：\ xampp \ htdocs \ Timetable \ projects \ TimetableAlgorithmn.php：27＆＃34;我尝试手动定义变量，但它仍然会出现相同的错误，所以我相信foreach循环很好。

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$link = mysqli_connect("localhost", "root", "","test");
$stmt =mysqli_prepare($link,"INSERT INTO timetable VALUES (?, ?, ?);");
foreach ($timetable as $day => $subTimetable) {
foreach ($subTimetable as $hour => $subject) {
    mysqli_stmt_bind_param($stmt,'iis', $Day, $hour, $subject);
    mysqli_stmt_execute($stmt);
}