diff --git a/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php b/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php
index 2b4e248407e64..74cba80f2e155 100644
--- a/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php
+++ b/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php
@@ -167,12 +167,11 @@ public function schedule(Message $iTipMessage) {
 		}
 
 		$mailto = $iTipMessage->recipient;
-		if($mailto === null) {
-			return;
-		}
-
 		// Strip off mailto:
 		$recipient = substr($mailto, 7);
+		if($recipient === false) {
+			return;
+		}
 		if (!$this->mailer->validateMailAddress($recipient)) {
 			// Nothing to send if the recipient doesn't have a valid email address
 			$iTipMessage->scheduleStatus = '5.0; EMail delivery failed';
@@ -218,6 +217,9 @@ public function schedule(Message $iTipMessage) {
 			$senderName = $this->userManager->getDisplayName($this->userId);
 		}
 		$sender = substr($iTipMessage->sender, 7);
+		if($sender === false) {
+			$sender = Util::getDefaultEmailAddress('invitations-noreply');
+		}
 
 		switch (strtolower($iTipMessage->method)) {
 			case self::METHOD_REPLY: