diff --git a/libraries/src/Mail/MailTemplate.php b/libraries/src/Mail/MailTemplate.php
index 605c44339d07d..b74c2b67d17f5 100644
--- a/libraries/src/Mail/MailTemplate.php
+++ b/libraries/src/Mail/MailTemplate.php
@@ -325,6 +325,11 @@ public function send()
     protected function replaceTags($text, $tags)
     {
         foreach ($tags as $key => $value) {
+            // If the value is NULL, replace with an empty string. NULL itself throws notices
+            if (\is_null($value)) {
+                $value = '';
+            }
+
             if (\is_array($value)) {
                 $matches = [];
                 $pregKey = preg_quote(strtoupper($key), '/');