diff --git a/Requirements.md b/Requirements.md
index d04f736..62f4638 100644
--- a/Requirements.md
+++ b/Requirements.md
@@ -9,6 +9,7 @@
   - [`simplexml`](https://secure.php.net/manual/en/book.simplexml.php) extension
   - [`curl`](https://secure.php.net/manual/en/book.curl.php) extension
   - [`json`](https://secure.php.net/manual/en/book.json.php) extension
+  - [`filter`](https://secure.php.net/manual/en/book.filter.php) extension
   - [`sqlite3`](http://php.net/manual/en/book.sqlite3.php) extension (only when using SQLiteCache)
 
 Enable extensions by un-commenting the corresponding line in your PHP configuration (`php.ini`).