diff --git a/includes/bootstrap.inc b/includes/bootstrap.inc
index 38d20a982d03de285ae52408dc8d37b31687506f..62329f851ed2ab3ee8c764f5a919deed37c9a927 100644
--- a/includes/bootstrap.inc
+++ b/includes/bootstrap.inc
@@ -1905,7 +1905,7 @@ function format_string($string, array $args = array()) {
  * @ingroup sanitization
  */
 function check_plain($text) {
-  return htmlspecialchars($text, ENT_QUOTES, 'UTF-8');
+  return htmlspecialchars((string) $text, ENT_QUOTES, 'UTF-8');
 }
 
 /**
diff --git a/modules/simpletest/tests/common.test b/modules/simpletest/tests/common.test
index b22c4a6f5ccab9d3a3b8f53ce57175c07f562298..3b4d731da3ca86da7a50aac5daf017283e4cd080 100644
--- a/modules/simpletest/tests/common.test
+++ b/modules/simpletest/tests/common.test
@@ -480,6 +480,11 @@ class CommonXssUnitTest extends DrupalUnitTestCase {
    * Check that invalid multi-byte sequences are rejected.
    */
   function testInvalidMultiByte() {
+     // Ignore PHP 8.0+ null deprecatations.
+     $text = check_plain(NULL);
+     $this->assertEqual($text, '', 'check_plain() casts null to string');
+     $text = check_plain(FALSE);
+     $this->assertEqual($text, '', 'check_plain() casts boolean to string');
      // Ignore PHP 5.3+ invalid multibyte sequence warning.
      $text = @check_plain("Foo\xC0barbaz");
      $this->assertEqual($text, '', 'check_plain() rejects invalid sequence "Foo\xC0barbaz"');