diff --git a/src/Message.php b/src/Message.php
index 7e8c43a..30fdc3b 100644
--- a/src/Message.php
+++ b/src/Message.php
@@ -324,7 +324,7 @@ public function getText(): ?string
      */
     public function setText(?string $text): void
     {
-        if (preg_match('//u', $text) !== 1) {
+        if (preg_match('//u', $text ?? '') !== 1) {
             throw new InvalidArgumentException("message body contains an invalid UTF-8 byte sequence");
         }
 
@@ -346,7 +346,7 @@ public function getHTML(): ?string
      */
     public function setHTML(?string $html): void
     {
-        if (preg_match('//u', $html) !== 1) {
+        if (preg_match('//u', $html ?? '') !== 1) {
             throw new InvalidArgumentException("message body contains an invalid UTF-8 byte sequence");
         }