diff --git a/src/Api/Ip.php b/src/Api/Ip.php
index dac8db8..80d4ab0 100644
--- a/src/Api/Ip.php
+++ b/src/Api/Ip.php
@@ -81,7 +81,7 @@ class Ip extends HttpApi
         Assert::ip($ip);
 
         $params = [
-            'id' => $ip,
+            'ip' => $ip,
         ];
 
         $response = $this->httpPost(sprintf('/v3/domains/%s/ips', $domain), $params);
diff --git a/tests/Api/IpTest.php b/tests/Api/IpTest.php
new file mode 100644
index 0000000..a319365
--- /dev/null
+++ b/tests/Api/IpTest.php
@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * Copyright (C) 2013 Mailgun
+ *
+ * This software may be modified and distributed under the terms
+ * of the MIT license. See the LICENSE file for details.
+ */
+
+namespace Mailgun\Tests\Api;
+
+use GuzzleHttp\Psr7\Response;
+use Mailgun\Api\Ip;
+use Mailgun\Model\Ip\UpdateResponse;
+
+class IpTest extends TestCase
+{
+    protected function getApiClass()
+    {
+        return Ip::class;
+    }
+
+    public function testAssign()
+    {
+        $this->setRequestMethod('POST');
+        $this->setRequestUri('/v3/domains/example.com/ips');
+        $this->setRequestBody([
+            'ip' => '127.0.0.1',
+        ]);
+        $this->setHydrateClass(UpdateResponse::class);
+
+        $api = $this->getApiInstance();
+        $api->assign('example.com', '127.0.0.1');
+    }
+}