@@ -87,7 +87,7 @@ protected function doSend(MessageInterface $message): SentMessage
 
 $response = $this->client->request('POST', $endpoint, [
 'auth_basic' => [$this->apiUsername, $this->apiKey],
-'json' => array_filter($options),
+'json' => ['messages' => [array_filter($options)]],
 ]);
 
 try {

@@ -63,10 +63,14 @@ public function testNoInvalidArgumentExceptionIsThrownIfFromIsValid(string $from
 $response = $this->createMock(ResponseInterface::class);
 $response->expects(self::exactly(2))->method('getStatusCode')->willReturn(200);
 $response->expects(self::once())->method('getContent')->willReturn('');
-$client = new MockHttpClient(function (string $method, string $url) use ($response): ResponseInterface {
+$client = new MockHttpClient(function (string $method, string $url, array $options) use ($response): ResponseInterface {
 self::assertSame('POST', $method);
 self::assertSame('https://rest.clicksend.com/v3/sms/send', $url);
 
+$body = json_decode($options['body'], true);
+self::assertIsArray($body);
+self::assertArrayHasKey('messages', $body);
+
 return $response;
 });
 $transport = $this->createTransport($client, $from);