INSERT INTO NULLABLE FIELDS THROWING ERRORS WITH NULL VALUES

[Shecky Van Sheck]

MySQL v5.6.20; Table in question:

CREATE TABLE `invoices` (
`INVOICE_NUMBER` smallint(4) unsigned NOT NULL AUTO_INCREMENT,
`INVOICE_DATE` date DEFAULT NULL,
`PROJECT_NUMBER` smallint(4) unsigned DEFAULT NULL,
`CLIENT` tinytext COLLATE utf8_unicode_ci,
`PROJECT_DESCRIPTION` tinytext COLLATE utf8_unicode_ci,
`TOTAL_HOURS` text COLLATE utf8_unicode_ci,
`SUBTOTAL` decimal(10,2) unsigned DEFAULT NULL,
`TAX` decimal(10,2) unsigned DEFAULT NULL,
`INVOICE_TOTAL` decimal(10,2) unsigned DEFAULT NULL,
`DATE_SENT` date DEFAULT NULL,
`DATE_RECEIVED` date DEFAULT NULL,
`NOTES` text COLLATE utf8_unicode_ci,
PRIMARY KEY (`INVOICE_NUMBER`)
) ENGINE=InnoDB AUTO_INCREMENT=1005 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

And the pertinent bits of my process.php are:

	if (isset($_POST['submit'])) {
		$invNum = $_POST['invNum'];
		$invDate = $_POST['invDate'];
		$projNum = $_POST['projNum'];
		$client = $_POST['client'];
		$projDesc = $_POST['projDesc'];
		$totalHours = $_POST['totalHours'];
		$subTotal = $_POST['subTotal'];
		$tax = $_POST['tax'];
		$invTotal = $_POST['invTotal'];
		$dateSent = $_POST['dateSent'];
		$dateReceived = $_POST['dateReceived'];
		$notes = $_POST['notes'];
		$notes = mysqli_real_escape_string($mysqli, $notes);

	echo "<div class=\"debug\">
			<table>
				<snip>( SUCCESSFULLY ECHOING ALL VARIABLES/VALUES )</snip>
			</table>
		</div>";

	$query = "INSERT INTO Invoices SET
		INVOICE_NUMBER = '$invNum',
		INVOICE_DATE = '$invDate',
		PROJECT_NUMBER = '$projNum',
		CLIENT = '$client',
		PROJECT_DESCRIPTION = '$projDesc',
		TOTAL_HOURS = '$totalHours',
		SUBTOTAL = '$subTotal',
		TAX = '$tax',
		INVOICE_TOTAL = '$invTotal',
		DATE_SENT = '$dateSent',
		DATE_RECEIVED = '$dateReceived',
		NOTES = '$notes'
		
		ON DUPLICATE KEY UPDATE
		INVOICE_NUMBER = VALUES(INVOICE_NUMBER),
		INVOICE_DATE = VALUES(INVOICE_DATE),
		PROJECT_NUMBER = VALUES(PROJECT_NUMBER),
		CLIENT = VALUES(CLIENT),
		PROJECT_DESCRIPTION = VALUES(PROJECT_DESCRIPTION),
		TOTAL_HOURS = VALUES(TOTAL_HOURS),
		SUBTOTAL = VALUES(SUBTOTAL),
		TAX = VALUES(TAX),
		INVOICE_TOTAL = VALUES(INVOICE_TOTAL),
		DATE_SENT = VALUES(DATE_SENT),
		DATE_RECEIVED = VALUES(DATE_RECEIVED),
		NOTES = VALUES(NOTES)
		";

Why am I getting errors when submitting my form with various fields empty, e.g.:
"INSERT failed: (1366) Incorrect decimal value: '' for column 'TAX' at row 1", or "INSERT failed: (1292) Incorrect date value: '' for column 'DATE_SENT' at row 1"

... despite the fact that - at least according to my newbie understanding, the definition of these fields should allow null (= empty?) values? Not surprisingly, the error will occur only on the first empty field, regardless of how many fields are empty. My understanding is that I should be able to execute an insert query with some empty fields; at some point in the recent past, I was able to do so, though I have recreated this table in the wake of other issues (and suspected corruption), but it seems that something in how I created or tweaked this table is the cause of this soul-crushing disappointment.

Also adding to my confusion is the fact that there is a second INSERT query running on the same page (process.php), following the one described above, which successfully inserts data into the Invoice_Line_Items table despite empty 'HOURS' field, which is defined thusly: (decimal(10,2) unsigned DEFAULT NULL); as far as I can tell, the problematic fields in the Invoices table are similarly defined.

Many thanks in advance,

svs