diff --git a/tests/phpunit/includes/class-jsonserializable-object.php b/tests/phpunit/includes/class-jsonserializable-object.php
index eb4ae5c3af..e235327020 100644
--- a/tests/phpunit/includes/class-jsonserializable-object.php
+++ b/tests/phpunit/includes/class-jsonserializable-object.php
@@ -15,6 +15,7 @@ class JsonSerializable_Object implements JsonSerializable {
 		$this->data = $data;
 	}
 
+	#[ReturnTypeWillChange]
 	public function jsonSerialize() {
 		return $this->data;
 	}