diff --git a/src/Application.php b/src/Application.php
index 821b279..fed6991 100644
--- a/src/Application.php
+++ b/src/Application.php
@@ -31,7 +31,7 @@ final class Application implements JsonSerializable {
         return $this->app_expiry ?? -1;
     }
 
-    public function jsonSerialize() {
+    public function jsonSerialize(): mixed {
         return [
             'id' => $this->getId(),
             'name' => $this->getName(),
diff --git a/src/Upload.php b/src/Upload.php
index b2b41ee..33fb954 100644
--- a/src/Upload.php
+++ b/src/Upload.php
@@ -196,7 +196,7 @@ final class Upload implements JsonSerializable {
         }
     }
 
-    public function jsonSerialize() {
+    public function jsonSerialize(): mixed {
         return [
             'id' => $this->getId(),
             'url' => $this->getPublicUrl(),
diff --git a/src/User.php b/src/User.php
index 84955c8..83a1ca6 100644
--- a/src/User.php
+++ b/src/User.php
@@ -47,7 +47,7 @@ class User implements JsonSerializable {
         return $this->getRestricted() > 0;
     }
 
-    public function jsonSerialize() {
+    public function jsonSerialize(): mixed {
         return [
             'id' => $this->getId(),
             'size_multi' => $this->getSizeMultiplier(),