diff --git a/phpunit-bootstrap.php b/phpunit-bootstrap.php
index 4064944..520bca7 100644
--- a/phpunit-bootstrap.php
+++ b/phpunit-bootstrap.php
@@ -98,5 +98,8 @@
 // Load test related autoloader.
 require_once __DIR__ . '/vendor/yoast/phpunit-polyfills/phpunitpolyfills-autoload.php';
 
+// Load IO testcase.
+require_once __DIR__ . '/Tests/IOTestCase.php';
+
 // Clean up.
 unset($ds, $phpcsDir, $phpcsUtilsDir, $vendorDir);