diff --git a/composer.json b/composer.json
index e2c5048..9b46196 100644
--- a/composer.json
+++ b/composer.json
@@ -23,7 +23,7 @@
     "vlucas/phpdotenv": "^5.5",
     "phpunit/phpunit": "^5.7 || ^9.0",
     "squizlabs/php_codesniffer": "^3.3",
-    "phpstan/phpstan": "^1.2"
+    "phpstan/phpstan": "^2.0"
   },
   "autoload": {
     "psr-4": {
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
index c3bd077..77033b3 100644
--- a/phpstan.neon.dist
+++ b/phpstan.neon.dist
@@ -6,4 +6,4 @@ parameters:
 
     # Should not need to edit anything below here
     # Rule Level: https://phpstan.org/user-guide/rule-levels
-    level: 8
\ No newline at end of file
+    level: 10
\ No newline at end of file
diff --git a/src/ConvertKit_API.php b/src/ConvertKit_API.php
index 25c843a..6fcd9d3 100644
--- a/src/ConvertKit_API.php
+++ b/src/ConvertKit_API.php
@@ -177,7 +177,7 @@ public function get_oauth_url(string $redirectURI)
      * @param string $authCode    Authorization Code, returned from get_oauth_url() flow.
      * @param string $redirectURI Redirect URI.
      *
-     * @return array<string, int|string> API response
+     * @return mixed|array<string, int|string> API response
      */
     public function get_access_token(string $authCode, string $redirectURI)
     {
@@ -215,7 +215,7 @@ public function get_access_token(string $authCode, string $redirectURI)
      * @param string $refreshToken Refresh Token.
      * @param string $redirectURI  Redirect URI.
      *
-     * @return array<string, int|string> API response
+     * @return mixed|array<string, int|string> API response
      */
     public function refresh_token(string $refreshToken, string $redirectURI)
     {
@@ -338,7 +338,7 @@ public function get_resource(string $url)
      *
      * @throws \Exception If JSON encoding arguments failed.
      *
-     * @return false|mixed
+     * @return mixed|object
      */
     public function request(string $endpoint, string $method, array $args = [])
     {
diff --git a/src/ConvertKit_API_Traits.php b/src/ConvertKit_API_Traits.php
index b5fc993..05773a3 100644
--- a/src/ConvertKit_API_Traits.php
+++ b/src/ConvertKit_API_Traits.php
@@ -161,7 +161,7 @@ public function get_growth_stats(\DateTime $starting = null, \DateTime $ending =
      *
      * @see https://developers.convertkit.com/v4.html#convertkit-api-forms
      *
-     * @return false|array<int,\stdClass>
+     * @return mixed|array<int,\stdClass>
      */
     public function get_forms(
         string $status = 'active',
@@ -198,7 +198,7 @@ public function get_forms(
      *
      * @see https://developers.convertkit.com/v4.html#convertkit-api-forms
      *
-     * @return false|array<int,\stdClass>
+     * @return mixed|array<int,\stdClass>
      */
     public function get_landing_pages(
         string $status = 'active',
@@ -456,7 +456,7 @@ public function get_sequence_subscriptions(
      *
      * @see https://developers.convertkit.com/v4.html#list-tags
      *
-     * @return false|array<int,\stdClass>
+     * @return mixed|array<int,\stdClass>
      */
     public function get_tags(
         bool $include_total_count = false,
@@ -863,10 +863,26 @@ public function get_subscriber_id(string $email_address)
             ['email_address' => $email_address]
         );
 
+        if (!$subscribers instanceof \stdClass) {
+            return false;
+        }
+
+        if (!is_array($subscribers->subscribers)) {
+            return false;
+        }
+
         if (!count($subscribers->subscribers)) {
             return false;
         }
 
+        if (!$subscribers->subscribers[0] instanceof \stdClass) {
+            return false;
+        }
+
+        if (!is_int($subscribers->subscribers[0]->id)) {
+            return false;
+        }
+
         // Return the subscriber's ID.
         return $subscribers->subscribers[0]->id;
     }
@@ -878,7 +894,7 @@ public function get_subscriber_id(string $email_address)
      *
      * @see https://developers.convertkit.com/v4.html#get-a-subscriber
      *
-     * @return false|integer
+     * @return mixed|integer
      */
     public function get_subscriber(int $subscriber_id)
     {
@@ -895,7 +911,7 @@ public function get_subscriber(int $subscriber_id)
      *
      * @see https://developers.convertkit.com/v4.html#update-a-subscriber
      *
-     * @return false|mixed
+     * @return mixed
      */
     public function update_subscriber(
         int $subscriber_id,
@@ -930,7 +946,7 @@ public function update_subscriber(
      *
      * @see https://developers.convertkit.com/v4.html#unsubscribe-subscriber
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function unsubscribe_by_email(string $email_address)
     {
@@ -949,7 +965,7 @@ public function unsubscribe_by_email(string $email_address)
      *
      * @see https://developers.convertkit.com/v4.html#unsubscribe-subscriber
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function unsubscribe(int $subscriber_id)
     {
@@ -967,7 +983,7 @@ public function unsubscribe(int $subscriber_id)
      *
      * @see https://developers.convertkit.com/v4.html#list-tags-for-a-subscriber
      *
-     * @return false|array<int,\stdClass>
+     * @return mixed|array<int,\stdClass>
      */
     public function get_subscriber_tags(
         int $subscriber_id,
@@ -1044,7 +1060,7 @@ public function get_broadcasts(
      *
      * @see https://developers.convertkit.com/v4.html#create-a-broadcast
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function create_broadcast(
         string $subject = '',
@@ -1103,7 +1119,7 @@ public function create_broadcast(
      *
      * @see https://developers.convertkit.com/v4.html#get-a-broadcast
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function get_broadcast(int $id)
     {
@@ -1118,7 +1134,7 @@ public function get_broadcast(int $id)
      *
      * @see https://developers.convertkit.com/v4.html#get-stats
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function get_broadcast_stats(int $id)
     {
@@ -1151,7 +1167,7 @@ public function get_broadcast_stats(int $id)
      *
      * @see https://developers.convertkit.com/#create-a-broadcast
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function update_broadcast(
         int $id,
@@ -1213,7 +1229,7 @@ public function update_broadcast(
      *
      * @see https://developers.convertkit.com/v4.html#delete-a-broadcast
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function delete_broadcast(int $id)
     {
@@ -1266,7 +1282,7 @@ public function get_webhooks(
      *
      * @throws \InvalidArgumentException If the event is not supported.
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function create_webhook(string $url, string $event, string $parameter = '')
     {
@@ -1340,7 +1356,7 @@ public function create_webhook(string $url, string $event, string $parameter = '
      *
      * @see https://developers.convertkit.com/v4.html#delete-a-webhook
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function delete_webhook(int $id)
     {
@@ -1389,7 +1405,7 @@ public function get_custom_fields(
      *
      * @see https://developers.convertkit.com/v4.html#create-a-custom-field
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function create_custom_field(string $label)
     {
@@ -1409,7 +1425,7 @@ public function create_custom_field(string $label)
      *
      * @see https://developers.convertkit.com/v4.html#bulk-create-custom-fields
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function create_custom_fields(array $labels, string $callback_url = '')
     {
@@ -1444,7 +1460,7 @@ public function create_custom_fields(array $labels, string $callback_url = '')
      *
      * @see https://developers.convertkit.com/v4.html#update-a-custom-field
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function update_custom_field(int $id, string $label)
     {
@@ -1463,7 +1479,7 @@ public function update_custom_field(int $id, string $label)
      *
      * @see https://developers.convertkit.com/#destroy-field
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function delete_custom_field(int $id)
     {
@@ -1510,7 +1526,7 @@ public function get_purchases(
      *
      * @see https://developers.convertkit.com/v4.html#get-a-purchase
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function get_purchase(int $purchase_id)
     {
@@ -1535,7 +1551,7 @@ public function get_purchase(int $purchase_id)
      *
      * @see https://developers.convertkit.com/v4.html#create-a-purchase
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function create_purchase(
         string $email_address,