diff --git a/system/Boot.php b/system/Boot.php
index 8b75908ded09..c38b6391f5e1 100644
--- a/system/Boot.php
+++ b/system/Boot.php
@@ -196,7 +196,13 @@ protected static function definePathConstants(Paths $paths): void
 
         // The path to the writable directory.
         if (! defined('WRITEPATH')) {
-            define('WRITEPATH', realpath(rtrim($paths->writableDirectory, '\\/ ')) . DIRECTORY_SEPARATOR);
+            if (($writePath = realpath(rtrim($paths->writableDirectory, '\\/ '))) === false) {
+                header('HTTP/1.1 503 Service Unavailable.', true, 503);
+                echo 'The WRITEPATH is not set correctly.';
+                // EXIT_ERROR is not yet defined
+                exit(1);
+            }
+            define('WRITEPATH', $writePath . DIRECTORY_SEPARATOR);
         }
 
         // The path to the tests directory