diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ae1ad66..a5aac08 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         operating-system: ['ubuntu-latest']
-        php-versions: ['7.4', '8.0']
+        php-versions: ['7.4', '8.0', '8.1']
         phpunit-versions: ['latest']
 
     steps:
diff --git a/src/Entity/Book.php b/src/Entity/Book.php
index 2562b53..db2eda2 100644
--- a/src/Entity/Book.php
+++ b/src/Entity/Book.php
@@ -140,7 +140,7 @@ public static function getAll(
     public static function getByFilters(
         Bookboon $bookboon,
         array $bookTypes = [self::_OWN_TYPE],
-        array $filters
+        array $filters = []
     ) : BookboonResponse {
         $variables = [
             'bookType' => join(',', $bookTypes),