diff --git a/src/Client/Response/CreateGroupResponse.php b/src/Client/Response/CreateGroupResponse.php
index 32daa47..8312dd9 100644
--- a/src/Client/Response/CreateGroupResponse.php
+++ b/src/Client/Response/CreateGroupResponse.php
@@ -25,7 +25,7 @@ class CreateGroupResponse extends Response
     /**
      * Obtained group key.
      */
-    private string $groupKey;
+    private ?string $groupKey = null;
 
     public function __construct(string $curlResponse)
     {
@@ -33,9 +33,9 @@ public function __construct(string $curlResponse)
     }
 
     /**
-     * @return string Obtained group key
+     * @return null|string Obtained group key or null if the request failed
      */
-    public function getGroupKey(): string
+    public function getGroupKey(): ?string
     {
         return $this->groupKey;
     }
diff --git a/src/Client/Response/LicenseResponse.php b/src/Client/Response/LicenseResponse.php
index f986d21..22db584 100644
--- a/src/Client/Response/LicenseResponse.php
+++ b/src/Client/Response/LicenseResponse.php
@@ -25,14 +25,17 @@ class LicenseResponse extends Response
     /**
      * Number of license credits remaining.
      */
-    private int $credits;
+    private ?int $credits = null;
 
     public function __construct(string $curlResponse)
     {
         $this->processCurlResponse($curlResponse);
     }
 
-    public function getCredits(): int
+    /**
+     * @return null|int License credits or null if the request failed
+     */
+    public function getCredits(): ?int
     {
         return $this->credits;
     }
diff --git a/src/Client/Response/RetrieveGroupResponse.php b/src/Client/Response/RetrieveGroupResponse.php
index 7166be2..eb83668 100644
--- a/src/Client/Response/RetrieveGroupResponse.php
+++ b/src/Client/Response/RetrieveGroupResponse.php
@@ -28,19 +28,22 @@ class RetrieveGroupResponse extends Response
     /**
      * Name of the group.
      */
-    private string $name;
+    private ?string $name = null;
 
     /**
      * @var Recipient[] Group users
      */
-    private array $users;
+    private array $users = [];
 
     public function __construct(string $curlResponse)
     {
         $this->processCurlResponse($curlResponse);
     }
 
-    public function getName(): string
+    /**
+     * @return null|string Group name or null if the request failed
+     */
+    public function getName(): ?string
     {
         return $this->name;
     }
diff --git a/src/Client/Response/SubscriptionResponse.php b/src/Client/Response/SubscriptionResponse.php
index 976b057..5f23bfe 100644
--- a/src/Client/Response/SubscriptionResponse.php
+++ b/src/Client/Response/SubscriptionResponse.php
@@ -25,14 +25,17 @@ class SubscriptionResponse extends Response
     /**
      * Applications that formerly collected Pushover user keys are encouraged to migrate to subscription keys.
      */
-    private string $subscribed_user_key;
+    private ?string $subscribed_user_key = null;
 
     public function __construct(string $curlResponse)
     {
         $this->processCurlResponse($curlResponse);
     }
 
-    public function getSubscribedUserKey(): string
+    /**
+     * @return null|string Subscribed user key or null if the request failed
+     */
+    public function getSubscribedUserKey(): ?string
     {
         return $this->subscribed_user_key;
     }
diff --git a/src/Client/Response/UserGroupValidationResponse.php b/src/Client/Response/UserGroupValidationResponse.php
index c235461..f69ce38 100644
--- a/src/Client/Response/UserGroupValidationResponse.php
+++ b/src/Client/Response/UserGroupValidationResponse.php
@@ -22,17 +22,17 @@
  */
 class UserGroupValidationResponse extends Response
 {
-    private bool $isGroup;
+    private ?bool $isGroup = null;
 
     /**
      * @var array<string>
      */
-    private array $devices;
+    private array $devices = [];
 
     /**
      * @var array<string>
      */
-    private array $licenses;
+    private array $licenses = [];
 
     public function __construct(string $curlResponse)
     {
@@ -60,41 +60,20 @@ public function getLicenses(): array
         return $this->licenses;
     }
 
-    private function setIsGroup(bool $isGroup): void
-    {
-        $this->isGroup = $isGroup;
-    }
-
-    /**
-     * @param array<string> $devices
-     */
-    private function setDevices(array $devices): void
-    {
-        $this->devices = $devices;
-    }
-
-    /**
-     * @param array<string> $licenses
-     */
-    private function setLicenses(array $licenses): void
-    {
-        $this->licenses = $licenses;
-    }
-
     private function processCurlResponse(string $curlResponse): void
     {
         $decodedCurlResponse = $this->processInitialCurlResponse($curlResponse);
 
         if ($this->getRequestStatus() === 1) {
-            $this->setDevices($decodedCurlResponse->devices);
-            $this->setLicenses($decodedCurlResponse->licenses);
+            $this->devices = $decodedCurlResponse->devices;
+            $this->licenses = $decodedCurlResponse->licenses;
 
             if ($decodedCurlResponse->group === 1) {
-                $this->setIsGroup(true);
+                $this->isGroup = true;
             }
 
             if ($decodedCurlResponse->group === 0) {
-                $this->setIsGroup(false);
+                $this->isGroup = false;
             }
         }
     }