diff --git a/CHANGELOG.md b/CHANGELOG.md
index ee4a985..d60af4c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,11 @@
 # Change Log
 
 The change log describes what is "Added", "Removed", "Changed" or "Fixed" between each release.
+## 3.5.5
+
+### Fixed
+- Cast integer values to string for sending multipart data
+
 ## 3.5.4
 
 ### Added
diff --git a/src/Api/Message.php b/src/Api/Message.php
index f804a49..28333d9 100644
--- a/src/Api/Message.php
+++ b/src/Api/Message.php
@@ -16,6 +16,7 @@ use Mailgun\Exception\InvalidArgumentException;
 use Mailgun\Message\BatchMessage;
 use Mailgun\Model\Message\SendResponse;
 use Mailgun\Model\Message\ShowResponse;
+use Psr\Http\Client\ClientExceptionInterface;
 use Psr\Http\Message\ResponseInterface;
 
 /**
@@ -34,6 +35,7 @@ class Message extends HttpApi
      * @see https://documentation.mailgun.com/en/latest/api-sending.html#sending
      *
      * @return SendResponse|ResponseInterface
+     * @throws \Exception|ClientExceptionInterface
      */
     public function send(string $domain, array $params)
     {
@@ -164,6 +166,9 @@ class Message extends HttpApi
         foreach ($params as $key => $value) {
             // If $value is not an array we cast it to an array
             foreach ((array) $value as $subValue) {
+                if (is_int($subValue)) {
+                    $subValue = (string)$subValue;
+                }
                 $postDataMultipart[] = [
                     'name' => $key,
                     'content' => $subValue,