diff --git a/lib/Db/LocalMessageMapper.php b/lib/Db/LocalMessageMapper.php
index 06ef03d023..0be085ce63 100644
--- a/lib/Db/LocalMessageMapper.php
+++ b/lib/Db/LocalMessageMapper.php
@@ -70,11 +70,19 @@ public function getAllForUser(string $userId, int $type = LocalMessage::TYPE_OUT
 
 		$recipientMap = [];
 		foreach ($recipients as $r) {
-			$recipientMap[$r->getLocalMessageId()][] = $r;
+			$localMessageId = $r->getLocalMessageId();
+			if ($localMessageId === null) {
+				continue;
+			}
+			$recipientMap[$localMessageId][] = $r;
 		}
 		$attachmentMap = [];
 		foreach ($attachments as $a) {
-			$attachmentMap[$a->getLocalMessageId()][] = $a;
+			$localMessageId = $a->getLocalMessageId();
+			if ($localMessageId === null) {
+				continue;
+			}
+			$attachmentMap[$localMessageId][] = $a;
 		}
 
 		return array_map(static function ($localMessage) use ($attachmentMap, $recipientMap) {
@@ -134,11 +142,19 @@ public function findDue(int $time, int $type = LocalMessage::TYPE_OUTGOING): arr
 
 		$recipientMap = [];
 		foreach ($recipients as $r) {
-			$recipientMap[$r->getLocalMessageId()][] = $r;
+			$localMessageId = $r->getLocalMessageId();
+			if ($localMessageId === null) {
+				continue;
+			}
+			$recipientMap[$localMessageId][] = $r;
 		}
 		$attachmentMap = [];
 		foreach ($attachments as $a) {
-			$attachmentMap[$a->getLocalMessageId()][] = $a;
+			$localMessageId = $a->getLocalMessageId();
+			if ($localMessageId === null) {
+				continue;
+			}
+			$attachmentMap[$localMessageId][] = $a;
 		}
 
 		return array_map(static function ($localMessage) use ($attachmentMap, $recipientMap) {
@@ -183,11 +199,19 @@ public function findDueDrafts(int $time): array {
 
 		$recipientMap = [];
 		foreach ($recipients as $r) {
-			$recipientMap[$r->getLocalMessageId()][] = $r;
+			$localMessageId = $r->getLocalMessageId();
+			if ($localMessageId === null) {
+				continue;
+			}
+			$recipientMap[$localMessageId][] = $r;
 		}
 		$attachmentMap = [];
 		foreach ($attachments as $a) {
-			$attachmentMap[$a->getLocalMessageId()][] = $a;
+			$localMessageId = $a->getLocalMessageId();
+			if ($localMessageId === null) {
+				continue;
+			}
+			$attachmentMap[$localMessageId][] = $a;
 		}
 
 		return array_map(static function ($localMessage) use ($attachmentMap, $recipientMap) {
@@ -232,7 +256,7 @@ public function updateWithRecipients(LocalMessage $message, array $to, array $cc
 		try {
 			$message = $this->update($message);
 
-			$this->recipientMapper->updateRecipients($message->getId(), $message->getRecipients(), $to, $cc, $bcc);
+			$this->recipientMapper->updateRecipients($message->getId(), $message->getRecipients() ?? [], $to, $cc, $bcc);
 			$recipients = $this->recipientMapper->findByLocalMessageId($message->getId());
 			$message->setRecipients($recipients);
 			$this->db->commit();
diff --git a/lib/Db/MessageMapper.php b/lib/Db/MessageMapper.php
index ce29c480ed..3a4e253207 100644
--- a/lib/Db/MessageMapper.php
+++ b/lib/Db/MessageMapper.php
@@ -531,7 +531,8 @@ public function updateBulk(Account $account, bool $permflagsEnabled, Message ...
 	 */
 	private function updateTags(Account $account, Message $message, array $tags, PerformanceLoggerTask $perf): void {
 		$imapTags = $message->getTags();
-		$dbTags = $tags[$message->getMessageId()] ?? [];
+		$messageId = $message->getMessageId();
+		$dbTags = $messageId !== null ? ($tags[$messageId] ?? []) : [];
 
 		if ($imapTags === [] && $dbTags === []) {
 			// neither old nor new tags
@@ -1340,7 +1341,8 @@ public function findRelatedData(array $messages, string $userId): array {
 		$tags = $this->tagMapper->getAllTagsForMessages($messages, $userId);
 		/** @var Message $message */
 		$messages = array_map(static function ($message) use ($tags) {
-			$message->setTags($tags[$message->getMessageId()] ?? []);
+			$messageId = $message->getMessageId();
+			$message->setTags($messageId !== null ? ($tags[$messageId] ?? []) : []);
 			return $message;
 		}, $messages);
 		return $messages;
diff --git a/lib/Http/JsonResponse.php b/lib/Http/JsonResponse.php
index e1a0300a6b..6d3b8143de 100644
--- a/lib/Http/JsonResponse.php
+++ b/lib/Http/JsonResponse.php
@@ -107,7 +107,7 @@ public static function error(string $message,
 	 * @param Http::STATUS_* $status
 	 * @param array|JsonSerializable|bool|string $data
 	 *
-	 * @return static
+	 * @return self
 	 */
 	public static function errorFromThrowable(Throwable $error,
 		int $status = Http::STATUS_INTERNAL_SERVER_ERROR,