diff --git a/src/Mailgun/Messages/MessageBuilder.php b/src/Mailgun/Messages/MessageBuilder.php
index 0fe1a1d..7e07f26 100644
--- a/src/Mailgun/Messages/MessageBuilder.php
+++ b/src/Mailgun/Messages/MessageBuilder.php
@@ -368,9 +368,9 @@ class MessageBuilder
     {
         if ($this->counters['attributes']['campaign_id'] < Api::CAMPAIGN_ID_LIMIT) {
             if (isset($this->message['o:campaign'])) {
-                array_push($this->message['o:campaign'], $campaignId);
+                array_push($this->message['o:campaign'], (string) $campaignId);
             } else {
-                $this->message['o:campaign'] = [$campaignId];
+                $this->message['o:campaign'] = [(string) $campaignId];
             }
             $this->counters['attributes']['campaign_id'] += 1;
 
diff --git a/tests/Messages/MessageBuilderTest.php b/tests/Messages/MessageBuilderTest.php
index 3414ec0..15b2d7e 100644
--- a/tests/Messages/MessageBuilderTest.php
+++ b/tests/Messages/MessageBuilderTest.php
@@ -245,7 +245,7 @@ class MessageBuilderTest extends \Mailgun\Tests\MailgunTestCase
         );
     }
 
-    public function testsetTestMode()
+    public function testSetTestMode()
     {
         $message = $this->client->MessageBuilder();
         $message->setTestMode(true);
@@ -262,17 +262,25 @@ class MessageBuilderTest extends \Mailgun\Tests\MailgunTestCase
         $this->assertEquals(['o:testmode' => 'no'], $messageObj);
     }
 
-    public function addCampaignId()
+    public function testAddCampaignId()
     {
         $message = $this->client->MessageBuilder();
         $message->addCampaignId('ABC123');
         $message->addCampaignId('XYZ987');
         $message->addCampaignId('TUV456');
-        $message->addCampaignId('NONO123');
         $messageObj = $message->getMessage();
         $this->assertEquals(['o:campaign' => ['ABC123', 'XYZ987', 'TUV456']], $messageObj);
     }
 
+    public function testAddCampaignIdInteger()
+    {
+        $message = $this->client->MessageBuilder();
+        $message->addCampaignId(1111);
+        $message->addCampaignId(2222);
+        $messageObj = $message->getMessage();
+        $this->assertSame(['o:campaign' => ['1111', '2222']], $messageObj);
+    }
+
     public function testSetDkim()
     {
         $message = $this->client->MessageBuilder();