diff --git a/src/JSON.php b/src/JSON.php
index fc3b98b1..ac9f8c62 100644
--- a/src/JSON.php
+++ b/src/JSON.php
@@ -196,7 +196,7 @@ public function resolveForDisplay($resource, $attribute = null)
      *
      * @return array
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return array_merge(parent::jsonSerialize(), [
             'panelTitleClasses' => $this->panelTitleClasses,