diff --git a/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php b/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php
index 74cba80f2e155..b7d1136d3085f 100644
--- a/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php
+++ b/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php
@@ -177,7 +177,7 @@ public function schedule(Message $iTipMessage) {
 			$iTipMessage->scheduleStatus = '5.0; EMail delivery failed';
 			return;
 		}
-		$recipientName = $iTipMessage->recipientName ?: null;
+		$recipientName = $iTipMessage->recipientName ? (string)$iTipMessage->recipientName : null;
 
 		$newEvents = $iTipMessage->message;
 		$oldEvents = $this->getVCalendar();
@@ -213,9 +213,11 @@ public function schedule(Message $iTipMessage) {
 			$senderName = $senderName->getValue() ?? null;
 		}
 
-		if ($senderName === null || empty(trim($senderName))) {
+		// Try to get the sender name from the current user id if available.
+		if ($this->userId !== null && ($senderName === null || empty(trim($senderName)))) {
 			$senderName = $this->userManager->getDisplayName($this->userId);
 		}
+
 		$sender = substr($iTipMessage->sender, 7);
 		if($sender === false) {
 			$sender = Util::getDefaultEmailAddress('invitations-noreply');
@@ -236,7 +238,6 @@ public function schedule(Message $iTipMessage) {
 				break;
 		}
 
-
 		$data['attendee_name'] = ($recipientName ?: $recipient);
 		$data['invitee_name'] = ($senderName ?: $sender);
 
@@ -244,9 +245,19 @@ public function schedule(Message $iTipMessage) {
 		$fromName = $this->imipService->getFrom($senderName, $this->defaults->getName());
 
 		$message = $this->mailer->createMessage()
-			->setFrom([$fromEMail => $fromName])
-			->setTo([$recipient => $recipientName ?: $recipient])
-			->setReplyTo([$sender => $senderName]);
+			->setFrom([$fromEMail => $fromName]);
+
+		if ($recipientName !== null) {
+			$message->setTo([$recipient => $recipientName]);
+		} else {
+			$message->setTo([$recipient]);
+		}
+
+		if ($senderName !== null) {
+			$message->setReplyTo([$sender => $senderName]);
+		} else {
+			$message->setReplyTo([$sender]);
+		}
 
 		$template = $this->mailer->createEMailTemplate('dav.calendarInvite.' . $method, $data);
 		$template->addHeader();
diff --git a/apps/dav/lib/CalDAV/Schedule/IMipService.php b/apps/dav/lib/CalDAV/Schedule/IMipService.php
index 88f8bb8f54e1c..531c1c8af170e 100644
--- a/apps/dav/lib/CalDAV/Schedule/IMipService.php
+++ b/apps/dav/lib/CalDAV/Schedule/IMipService.php
@@ -70,11 +70,15 @@ public function __construct(URLGenerator $urlGenerator,
 	}
 
 	/**
-	 * @param string $senderName
-	 * @param $default
+	 * @param string|null $senderName
+	 * @param string $default
 	 * @return string
 	 */
-	public function getFrom(string $senderName, $default): string {
+	public function getFrom(?string $senderName, string $default): string {
+		if ($senderName === null) {
+			return $default;
+		}
+
 		return $this->l10n->t('%1$s via %2$s', [$senderName, $default]);
 	}