diff --git a/lib/Db/AclMapper.php b/lib/Db/AclMapper.php
index 732fb54f8..96aad94f6 100644
--- a/lib/Db/AclMapper.php
+++ b/lib/Db/AclMapper.php
@@ -23,6 +23,8 @@
 
 namespace OCA\Deck\Db;
 
+use OCP\AppFramework\Db\DoesNotExistException;
+use OCP\AppFramework\Db\MultipleObjectsReturnedException;
 use OCP\IDBConnection;
 
 class AclMapper extends DeckMapper implements IPermissionMapper {
@@ -43,8 +45,12 @@ class AclMapper extends DeckMapper implements IPermissionMapper {
 	}
 
 	public function findBoardId($aclId): ?int {
-		$entity = $this->find($aclId);
-		return $entity->getBoardId();
+		try {
+			$entity = $this->find($aclId);
+			return $entity->getBoardId();
+		} catch (DoesNotExistException | MultipleObjectsReturnedException $e) {
+		}
+		return null;
 	}
 
 	public function findByParticipant($type, $participant): array {
diff --git a/lib/Db/CardMapper.php b/lib/Db/CardMapper.php
index 8866137a0..461a93611 100644
--- a/lib/Db/CardMapper.php
+++ b/lib/Db/CardMapper.php
@@ -322,8 +322,7 @@ class CardMapper extends QBMapper implements IPermissionMapper {
 		$stmt = $this->db->prepare($sql);
 		$stmt->bindParam(1, $cardId, \PDO::PARAM_INT);
 		$stmt->execute();
-		$row = $stmt->fetch();
-		return $row['id'];
+		return $stmt->fetchColumn() ?? null;
 	}
 
 	public function mapOwner(Card &$card) {
diff --git a/lib/Db/LabelMapper.php b/lib/Db/LabelMapper.php
index 26f89bf3a..2f3c2ad4c 100644
--- a/lib/Db/LabelMapper.php
+++ b/lib/Db/LabelMapper.php
@@ -23,7 +23,9 @@
 
 namespace OCA\Deck\Db;
 
+use OCP\AppFramework\Db\DoesNotExistException;
 use OCP\AppFramework\Db\Entity;
+use OCP\AppFramework\Db\MultipleObjectsReturnedException;
 use OCP\IDBConnection;
 
 class LabelMapper extends DeckMapper implements IPermissionMapper {
@@ -100,7 +102,12 @@ class LabelMapper extends DeckMapper implements IPermissionMapper {
 	}
 
 	public function findBoardId($labelId): ?int {
-		$entity = $this->find($labelId);
-		return $entity->getBoardId();
+		try {
+			$entity = $this->find($labelId);
+			return $entity->getBoardId();
+		} catch (DoesNotExistException $e) {
+		} catch (MultipleObjectsReturnedException $e) {
+		}
+		return null;
 	}
 }
diff --git a/lib/Db/StackMapper.php b/lib/Db/StackMapper.php
index 88cffeeba..8cf28918a 100644
--- a/lib/Db/StackMapper.php
+++ b/lib/Db/StackMapper.php
@@ -23,7 +23,9 @@
 
 namespace OCA\Deck\Db;
 
+use OCP\AppFramework\Db\DoesNotExistException;
 use OCP\AppFramework\Db\Entity;
+use OCP\AppFramework\Db\MultipleObjectsReturnedException;
 use OCP\IDBConnection;
 
 class StackMapper extends DeckMapper implements IPermissionMapper {
@@ -75,7 +77,12 @@ class StackMapper extends DeckMapper implements IPermissionMapper {
 	}
 
 	public function findBoardId($stackId): ?int {
-		$entity = $this->find($stackId);
-		return $entity->getBoardId();
+		try {
+			$entity = $this->find($stackId);
+			return $entity->getBoardId();
+		} catch (DoesNotExistException $e) {
+		} catch (MultipleObjectsReturnedException $e) {
+		}
+		return null;
 	}
 }
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())