diff --git a/Tests/Functional/Fixtures/Traits/SiteBasedTestTrait.php b/Tests/Functional/Fixtures/Traits/SiteBasedTestTrait.php
index 0c67890d..7a243c6a 100644
--- a/Tests/Functional/Fixtures/Traits/SiteBasedTestTrait.php
+++ b/Tests/Functional/Fixtures/Traits/SiteBasedTestTrait.php
@@ -119,7 +119,7 @@ protected function buildLanguageConfiguration(
         string $identifier,
         string $base,
         array $fallbackIdentifiers = [],
-        string $fallbackType = null
+        ?string $fallbackType = null
     ): array {
         $preset = $this->resolveLanguagePreset($identifier);