diff --git a/lib/Model/AbstractModel.php b/lib/Model/AbstractModel.php
index d1daff14..9c2e63ab 100644
--- a/lib/Model/AbstractModel.php
+++ b/lib/Model/AbstractModel.php
@@ -13,10 +13,8 @@ abstract class AbstractModel
 {
     /**
      * Gets the string presentation of the object.
-     *
-     * @return string
      */
-    public function __toString()
+    public function __toString(): string
     {
         return json_encode(
             ObjectSerializer::sanitizeForSerialization($this),
@@ -32,17 +30,15 @@ public function __toString()
      * @return mixed returns data which can be serialized by json_encode(), which is a value
      *               of any type other than a resource
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         return ObjectSerializer::sanitizeForSerialization($this);
     }
 
     /**
      * Gets a header-safe presentation of the object.
-     *
-     * @return string
      */
-    public function toHeaderValue()
+    public function toHeaderValue(): string
     {
         return json_encode(ObjectSerializer::sanitizeForSerialization($this));
     }