diff --git a/.env.dist.testing b/.env.dist.testing
index f7cfb50..6e6f963 100644
--- a/.env.dist.testing
+++ b/.env.dist.testing
@@ -4,6 +4,7 @@ CONVERTKIT_API_FORM_ID="2765139"
 CONVERTKIT_API_FORM_ID_2="2780977"
 CONVERTKIT_API_LEGACY_FORM_URL="https://app.convertkit.com/landing_pages/470099"
 CONVERTKIT_API_LANDING_PAGE_URL="https://cheerful-architect-3237.ck.page/cc5eb21744"
+CONVERTKIT_API_LANDING_PAGE_CHARACTER_ENCODING_URL="https://cheerful-architect-3237.ck.page/cc5eb21744"
 CONVERTKIT_API_LEGACY_LANDING_PAGE_URL="https://app.convertkit.com/landing_pages/470103"
 CONVERTKIT_API_SEQUENCE_ID="1030824"
 CONVERTKIT_API_SEQUENCE_EMAIL_ID="4533458"
diff --git a/.env.example b/.env.example
index 7cc34b4..09e8a97 100644
--- a/.env.example
+++ b/.env.example
@@ -16,6 +16,7 @@ CONVERTKIT_API_FORM_ID="2765139"
 CONVERTKIT_API_FORM_ID_2="2780977"
 CONVERTKIT_API_LEGACY_FORM_URL="https://app.convertkit.com/landing_pages/470099"
 CONVERTKIT_API_LANDING_PAGE_URL="https://cheerful-architect-3237.ck.page/cc5eb21744"
+CONVERTKIT_API_LANDING_PAGE_CHARACTER_ENCODING_URL="https://cheerful-architect-3237.ck.page/cc5eb21744"
 CONVERTKIT_API_LEGACY_LANDING_PAGE_URL="https://app.convertkit.com/landing_pages/470103"
 CONVERTKIT_API_SEQUENCE_ID="1030824"
 CONVERTKIT_API_SEQUENCE_EMAIL_ID="4533458"
diff --git a/phpunit.xml b/phpunit.xml
index dd54dd6..0d14407 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -3,7 +3,6 @@
     <testsuites>
         <testsuite name="ConvertKit API Tests">
             <directory>tests</directory>
-            <exclude>tests/ConvertKitAPITest.php</exclude>
         </testsuite>
     </testsuites>
 </phpunit>
diff --git a/tests/ConvertKitAPITest.php b/tests/ConvertKitAPITest.php
index 93a4b13..c7d7c3f 100644
--- a/tests/ConvertKitAPITest.php
+++ b/tests/ConvertKitAPITest.php
@@ -14,8 +14,11 @@
 
 /**
  * ConvertKit API class tests.
+ *
+ * Extended by ConvertKitAPIKeyTest and ConvertKitAPIOAuthTest, which define
+ * the authentication method to use when running these tests.
  */
-class ConvertKitAPITest extends TestCase
+abstract class ConvertKitAPITest extends TestCase
 {
     use TestsTrait;
 
@@ -102,18 +105,29 @@ protected function tearDown(): void
     }
 
     /**
-     * Assert that the given callable throws a ClientException, ServerException, or InvalidArgumentException.
+     * Assert that the given callable throws an exception.
+     *
+     * Any Throwable is accepted by default, as the API may return a ClientException
+     * or a ServerException depending on the error. Where the SDK validates arguments
+     * before performing an API request, specify $expected, to assert that the SDK's
+     * validation produced the error, and not the API.
      *
      * @since   2.7.0
      *
-     * @param   callable $fn Callable that should fail.
+     * @param   callable    $fn       Callable that should fail.
+     * @param   string|null $expected Expected exception class name.
      * @return  void
      */
-    protected function assertApiError(callable $fn): void
+    protected function assertApiError(callable $fn, string|null $expected = null): void
     {
         try {
             $fn();
         } catch (\Throwable $e) {
+            if (!is_null($expected)) {
+                $this->assertInstanceOf($expected, $e);
+                return;
+            }
+
             $this->assertTrue(true, 'Callable threw an exception as expected.');
             return;
         }
diff --git a/tests/TestsTrait.php b/tests/TestsTrait.php
index 1bc0cfb..0f14ad0 100644
--- a/tests/TestsTrait.php
+++ b/tests/TestsTrait.php
@@ -6161,12 +6161,15 @@ public function testCreateWebhookWithEventParameter()
      */
     public function testCreateWebhookWithInvalidEvent()
     {
-        $this->assertApiError(function () {
-            return $this->api->create_webhook(
-                url: 'https://webhook.site/' . str_shuffle('wfervdrtgsdewrafvwefds'),
-                event: 'invalid.event'
-            );
-        });
+        $this->assertApiError(
+            function () {
+                return $this->api->create_webhook(
+                    url: 'https://webhook.site/' . str_shuffle('wfervdrtgsdewrafvwefds'),
+                    event: 'invalid.event'
+                );
+            },
+            \InvalidArgumentException::class
+        );
     }
 
     /**
@@ -6577,7 +6580,7 @@ public function testGetPurchasesPagination()
     }
 
     /**
-     * Test that get_purchases() returns the expected data.
+     * Test that get_purchase() returns the expected data.
      *
      * @since   1.0.0
      *
@@ -6863,7 +6866,7 @@ public function testGetResourceLegacyForm()
      */
     public function testGetResourceLandingPage()
     {
-        $markup = $this->api->get_resource($_ENV['CONVERTKIT_API_LANDING_PAGE_URL']);
+        $markup = $this->api->get_resource($_ENV['CONVERTKIT_API_LANDING_PAGE_CHARACTER_ENCODING_URL']);
 
         // Assert that the markup is HTML.
         $this->assertTrue($this->isHtml($markup));
@@ -6900,9 +6903,12 @@ public function testGetResourceLegacyLandingPage()
      */
     public function testGetResourceInvalidURL()
     {
-        $this->assertApiError(function () {
-            return $this->api->get_resource('not-a-url');
-        });
+        $this->assertApiError(
+            function () {
+                return $this->api->get_resource('not-a-url');
+            },
+            \InvalidArgumentException::class
+        );
     }
 
     /**
@@ -6954,14 +6960,18 @@ public function isHtml($string)
     /**
      * Helper method to assert the given key exists as an array in the API response.
      *
+     * Accepts either a stdClass object (PHP SDK, Guzzle-decoded) or an
+     * associative array (WP Libs, wp_remote_retrieve_body -> json_decode true),
+     * so the same trait file works verbatim in both repos.
+     *
      * @since   2.0.0
      *
-     * @param   object $result API Result.
-     * @param   string $key    Key.
+     * @param   object|array<string, mixed> $result API Result.
+     * @param   string                      $key    Key.
      */
     public function assertDataExists($result, $key)
     {
-        $result = get_object_vars($result);
+        $result = is_object($result) ? get_object_vars($result) : $result;
         $this->assertArrayHasKey($key, $result);
         $this->assertIsArray($result[$key]);
     }
@@ -6969,15 +6979,18 @@ public function assertDataExists($result, $key)
     /**
      * Helper method to assert pagination object exists in response.
      *
+     * Accepts either a stdClass object (PHP SDK) or an associative array
+     * (WP Libs), so the same trait file works verbatim in both repos.
+     *
      * @since   2.0.0
      *
-     * @param   object $result API Result.
+     * @param   object|array<string, mixed> $result API Result.
      */
     public function assertPaginationExists($result)
     {
-        $result = get_object_vars($result);
+        $result     = is_object($result) ? get_object_vars($result) : $result;
         $this->assertArrayHasKey('pagination', $result);
-        $pagination = get_object_vars($result['pagination']);
+        $pagination = is_object($result['pagination']) ? get_object_vars($result['pagination']) : $result['pagination'];
         $this->assertArrayHasKey('has_previous_page', $pagination);
         $this->assertArrayHasKey('has_next_page', $pagination);
         $this->assertArrayHasKey('start_cursor', $pagination);