diff --git a/src/NovaBusyResourceField.php b/src/NovaBusyResourceField.php
index ed3ebb0..33f930b 100644
--- a/src/NovaBusyResourceField.php
+++ b/src/NovaBusyResourceField.php
@@ -20,7 +20,7 @@ class NovaBusyResourceField extends Field
      * @param  string|null  $attribute
      * @return void
      */
-    public function resolve($resource, $attribute = null)
+    public function resolve($resource, ?string $attribute = null): void
     {
         parent::resolve($resource, $attribute);