diff --git a/src/Api/MailingList/Member.php b/src/Api/MailingList/Member.php
index 742687b..5968c9e 100644
--- a/src/Api/MailingList/Member.php
+++ b/src/Api/MailingList/Member.php
@@ -100,7 +100,7 @@ class Member extends HttpApi
         $params = [
             'address' => $address,
             'name' => $name,
-            'vars' => $vars,
+            'vars' => \json_encode($vars),
             'subscribed' => $subscribed ? 'yes' : 'no',
             'upsert' => $upsert ? 'yes' : 'no',
         ];
@@ -144,19 +144,21 @@ class Member extends HttpApi
 
             Assert::isArray($data);
 
-            foreach ($data as $field => $value) {
+            foreach ($data as $field => &$value) {
                 switch ($field) {
                     case 'address':
                         Assert::stringNotEmpty($value);
 
                         break;
+                    case 'vars':
+                        if (is_array($value)) {
+                            $value = json_encode($value);
+                        }
+                        // We should assert that "vars"'s $value is a string.
+                        // no break
                     case 'name':
                         Assert::string($value);
 
-                        break;
-                    case 'vars':
-                        Assert::isArray($value);
-
                         break;
                     case 'subscribed':
                         Assert::oneOf($value, ['yes', 'no']);
@@ -195,14 +197,16 @@ class Member extends HttpApi
 
         foreach ($parameters as $field => $value) {
             switch ($field) {
+                case 'vars':
+                    if (is_array($value)) {
+                        $value = json_encode($value);
+                    }
+                    // We should assert that "vars"'s $value is a string.
+                    // no break
                 case 'address':
                 case 'name':
                     Assert::stringNotEmpty($value);
 
-                    break;
-                case 'vars':
-                    Assert::isArray($value);
-
                     break;
                 case 'subscribed':
                     Assert::oneOf($value, ['yes', 'no']);
diff --git a/src/Api/Route.php b/src/Api/Route.php
index 859a6c2..8081a2c 100644
--- a/src/Api/Route.php
+++ b/src/Api/Route.php
@@ -80,7 +80,7 @@ class Route extends HttpApi
         Assert::isArray($actions);
 
         $params = [
-            'priority' => $priority,
+            'priority' => (string) $priority,
             'expression' => $expression,
             'action' => $actions,
             'description' => $description,
@@ -129,7 +129,7 @@ class Route extends HttpApi
         }
 
         if (!empty($priority)) {
-            $params['priority'] = $priority;
+            $params['priority'] = (string) $priority;
         }
 
         $response = $this->httpPut(sprintf('/v3/routes/%s', $routeId), $params);
diff --git a/src/HttpClient/RequestBuilder.php b/src/HttpClient/RequestBuilder.php
index ed0c1d4..827ad5f 100644
--- a/src/HttpClient/RequestBuilder.php
+++ b/src/HttpClient/RequestBuilder.php
@@ -53,7 +53,7 @@ class RequestBuilder
     public function create(string $method, string $uri, array $headers = [], $body = null): RequestInterface
     {
         if (!is_array($body)) {
-            $stream = $this->getStreamFactory()->createStream($body);
+            $stream = $this->getStreamFactory()->createStream((string) $body);
 
             return $this->createRequest($method, $uri, $headers, $stream);
         }
diff --git a/src/Mailgun.php b/src/Mailgun.php
index 75a2362..ae12b7a 100644
--- a/src/Mailgun.php
+++ b/src/Mailgun.php
@@ -23,7 +23,7 @@ use Psr\Http\Client\ClientInterface;
 /**
  * This class is the base class for the Mailgun SDK.
  */
-final class Mailgun
+class Mailgun
 {
     /**
      * @var string|null
diff --git a/tests/Api/MailingList/MemberTest.php b/tests/Api/MailingList/MemberTest.php
index 9157468..8f5f3a6 100644
--- a/tests/Api/MailingList/MemberTest.php
+++ b/tests/Api/MailingList/MemberTest.php
@@ -70,7 +70,7 @@ class MemberTest extends TestCase
         $data = [
             'address' => 'foo@example.com',
             'name' => 'Foo',
-            'vars' => [],
+            'vars' => \json_encode([]),
             'subscribed' => 'yes',
             'upsert' => 'no',
         ];
@@ -162,9 +162,9 @@ class MemberTest extends TestCase
     public function testUpdate()
     {
         $data = [
-            'vars' => [
+            'vars' => \json_encode([
                 'foo' => 'bar',
-            ],
+            ]),
             'subscribed' => 'yes',
         ];
 
@@ -182,7 +182,7 @@ class MemberTest extends TestCase
         $this->expectException(InvalidArgumentException::class);
 
         $data = [
-            'vars' => 'foo=bar',
+            'vars' => 4711,
             'subscribed' => 'yes',
         ];
 
diff --git a/tests/Api/RouteTest.php b/tests/Api/RouteTest.php
index b5e547f..b91d18a 100644
--- a/tests/Api/RouteTest.php
+++ b/tests/Api/RouteTest.php
@@ -63,7 +63,7 @@ class RouteTest extends TestCase
             'expression' => 'catch_all()',
            'action' => 'forward("mailbox@myapp.com")',
            'description' => 'example',
-           'priority' => 100,
+           'priority' => '100',
         ]);
 
         $api = $this->getApiInstance();