diff --git a/src/Factory/Api/Mastodon/Attachment.php b/src/Factory/Api/Mastodon/Attachment.php
index 5a87aeed95..accebe3433 100644
--- a/src/Factory/Api/Mastodon/Attachment.php
+++ b/src/Factory/Api/Mastodon/Attachment.php
@@ -94,12 +94,17 @@ class Attachment extends BaseFactory
 	 */
 	public function createFromPhoto(int $id): array
 	{
-		$photo = Photo::selectFirst(['resource-id', 'uid', 'id', 'title', 'type'], ['id' => $id]);
+		$photo = Photo::selectFirst(['resource-id', 'uid', 'id', 'title', 'type', 'width', 'height'], ['id' => $id]);
 		if (empty($photo)) {
 			return [];
 		}
 
-		$attachment = ['id' => $photo['id'], 'description' => $photo['title']];
+		$attachment = [
+			'id'          => $photo['id'],
+			'description' => $photo['title'],
+			'width'       => $photo['width'],
+			'height'      => $photo['height'],
+		];
 
 		$photoTypes = Images::supportedTypes();
 		$ext        = $photoTypes[$photo['type']];
@@ -113,7 +118,6 @@ class Attachment extends BaseFactory
 			$preview_url = '';
 		}
 
-
 		$object = new \Friendica\Object\Api\Mastodon\Attachment($attachment, 'image', $url, $preview_url, '');
 		return $object->toArray();
 	}
diff --git a/src/Factory/Api/Mastodon/Notification.php b/src/Factory/Api/Mastodon/Notification.php
index adaf2d8689..32ea681dae 100644
--- a/src/Factory/Api/Mastodon/Notification.php
+++ b/src/Factory/Api/Mastodon/Notification.php
@@ -87,6 +87,11 @@ class Notification extends BaseFactory
 			if (($contact['uid'] == 0) && !empty($contact['uri-id'])) {
 				$contact = Contact::selectFirst(['pending'], ['uri-id' => $contact['uri-id'], 'uid' => $Notification->uid]);
 			}
+
+			if (!isset($contact['pending'])) {
+				return '';
+			}
+
 			$type = $contact['pending'] ? MstdnNotification::TYPE_INTRODUCTION : MstdnNotification::TYPE_FOLLOW;
 		} elseif (($Notification->verb == Activity::ANNOUNCE) &&
 			in_array($Notification->type, [Post\UserNotification::TYPE_DIRECT_COMMENT, Post\UserNotification::TYPE_DIRECT_THREAD_COMMENT])) {
diff --git a/src/Object/Api/Mastodon/Attachment.php b/src/Object/Api/Mastodon/Attachment.php
index 90b1bffb9f..3f890bf744 100644
--- a/src/Object/Api/Mastodon/Attachment.php
+++ b/src/Object/Api/Mastodon/Attachment.php
@@ -22,6 +22,8 @@
 namespace Friendica\Object\Api\Mastodon;
 
 use Friendica\BaseDataTransferObject;
+use Friendica\Core\Logger;
+use Friendica\Core\System;
 
 /**
  * Class Attachment
@@ -50,8 +52,12 @@ class Attachment extends BaseDataTransferObject
 	/**
 	 * Creates an attachment
 	 *
-	 * @param array $attachment
-	 * @throws \Friendica\Network\HTTPException\InternalServerErrorException
+	 * @param array $attachment Expected keys: id, description
+	 *                          If $type == 'image': width, height[, preview-width, preview-height]
+	 * @param string $type      One of: audio, video, gifv, image, unknown
+	 * @param string $url
+	 * @param string $preview
+	 * @param string $remote
 	 */
 	public function __construct(array $attachment, string $type, string $url, string $preview, string $remote)
 	{
@@ -70,7 +76,7 @@ class Attachment extends BaseDataTransferObject
 				$this->meta['original']['aspect'] = (float) ((int)  $attachment['width'] / (int) $attachment['height']);
 			}
 
-			if ((int) $attachment['preview-width'] > 0 && (int) $attachment['preview-height'] > 0) {
+			if (isset($attachment['preview-width']) && (int) $attachment['preview-width'] > 0 && (int) $attachment['preview-height'] > 0) {
 				$this->meta['small']['width'] = (int) $attachment['preview-width'];
 				$this->meta['small']['height'] = (int) $attachment['preview-height'];
 				$this->meta['small']['size'] = (int) $attachment['preview-width'] . 'x' . (int) $attachment['preview-height'];
diff --git a/src/Protocol/ActivityPub/Processor.php b/src/Protocol/ActivityPub/Processor.php
index 98fed12a33..3f61fe8f12 100644
--- a/src/Protocol/ActivityPub/Processor.php
+++ b/src/Protocol/ActivityPub/Processor.php
@@ -431,7 +431,7 @@ class Processor
 			$item['owner-id'] = $item['author-id'];
 		} else {
 			$actor = APContact::getByURL($item['owner-link'], false);
-			$item['isForum'] = ($actor['type'] == 'Group');
+			$item['isForum'] = ($actor['type'] ?? 'Person') == 'Group';
 		}
 
 		$item['uri'] = $activity['id'];
diff --git a/src/Protocol/ActivityPub/Transmitter.php b/src/Protocol/ActivityPub/Transmitter.php
index 1d71e93337..65abb5d5dd 100644
--- a/src/Protocol/ActivityPub/Transmitter.php
+++ b/src/Protocol/ActivityPub/Transmitter.php
@@ -2031,6 +2031,10 @@ class Transmitter
 		}
 
 		$owner = User::getOwnerDataById($uid);
+		if (empty($owner)) {
+			Logger::warning('No user found for actor, aborting', ['uid' => $uid]);
+			return false;
+		}
 
 		if (empty($id)) {
 			$id = DI::baseUrl() . '/activity/' . System::createGUID();
@@ -2167,6 +2171,11 @@ class Transmitter
 		}
 
 		$owner = User::getOwnerDataById($uid);
+		if (empty($owner)) {
+			Logger::notice('No user found for actor', ['uid' => $uid]);
+			return false;
+		}
+
 		$data = [
 			'@context' => ActivityPub::CONTEXT,
 			'id' => DI::baseUrl() . '/activity/' . System::createGUID(),