diff --git a/src/BootstrapRenderer.php b/src/BootstrapRenderer.php
index 840eeb6..923c4c8 100644
--- a/src/BootstrapRenderer.php
+++ b/src/BootstrapRenderer.php
@@ -14,7 +14,7 @@
 
 class BootstrapRenderer extends DefaultFormRenderer
 {
-    public $wrappers = [
+    public array $wrappers = [
         'form' => [
             'container' => null,
         ],