diff --git a/lib/Notification/Notifier.php b/lib/Notification/Notifier.php
index 98c97dc5f..3ea422cf5 100644
--- a/lib/Notification/Notifier.php
+++ b/lib/Notification/Notifier.php
@@ -28,6 +28,7 @@ use OCA\Deck\Db\CardMapper;
 use OCP\IURLGenerator;
 use OCP\IUserManager;
 use OCP\L10N\IFactory;
+use OCP\Notification\AlreadyProcessedException;
 use OCP\Notification\INotification;
 use OCP\Notification\INotifier;
 
@@ -96,6 +97,9 @@ class Notifier implements INotifier {
 			case 'card-assigned':
 				$cardId = $notification->getObjectId();
 				$boardId = $this->cardMapper->findBoardId($cardId);
+				if (!$boardId) {
+					throw new AlreadyProcessedException();
+				}
 				$initiator = $this->userManager->get($params[2]);
 				if ($initiator !== null) {
 					$dn = $initiator->getDisplayName();
@@ -120,6 +124,9 @@ class Notifier implements INotifier {
 			case 'card-overdue':
 				$cardId = $notification->getObjectId();
 				$boardId = $this->cardMapper->findBoardId($cardId);
+				if (!$boardId) {
+					throw new AlreadyProcessedException();
+				}
 				$notification->setParsedSubject(
 					(string) $l->t('The card "%s" on "%s" has reached its due date.', $params)
 				);
@@ -128,6 +135,9 @@ class Notifier implements INotifier {
 			case 'card-comment-mentioned':
 				$cardId = $notification->getObjectId();
 				$boardId = $this->cardMapper->findBoardId($cardId);
+				if (!$boardId) {
+					throw new AlreadyProcessedException();
+				}
 				$initiator = $this->userManager->get($params[2]);
 				if ($initiator !== null) {
 					$dn = $initiator->getDisplayName();
@@ -154,6 +164,9 @@ class Notifier implements INotifier {
 				break;
 			case 'board-shared':
 				$boardId = $notification->getObjectId();
+				if (!$boardId) {
+					throw new AlreadyProcessedException();
+				}
 				$initiator = $this->userManager->get($params[1]);
 				if ($initiator !== null) {
 					$dn = $initiator->getDisplayName();
diff --git a/tests/unit/Notification/NotifierTest.php b/tests/unit/Notification/NotifierTest.php
index 925ac813e..3acc7fd7c 100644
--- a/tests/unit/Notification/NotifierTest.php
+++ b/tests/unit/Notification/NotifierTest.php
@@ -177,6 +177,10 @@ class NotifierTest extends \Test\TestCase {
 
 	/** @dataProvider dataPrepareCardAssigned */
 	public function testPrepareCardAssigned($withUserFound = true) {
+		$this->cardMapper->expects($this->once())
+			->method('findBoardId')
+			->willReturn(123);
+
 		/** @var INotification $notification */
 		$notification = $this->createMock(INotification::class);
 		$notification->expects($this->once())