diff --git a/src/Curl/Curl.php b/src/Curl/Curl.php
index 7beae7a..e29592f 100644
--- a/src/Curl/Curl.php
+++ b/src/Curl/Curl.php
@@ -207,7 +207,7 @@ class Curl
                 // php_raw_url_encode()
                 // php_url_encode()
                 // https://github.com/php/php-src/blob/master/ext/standard/http.c
-                return urlencode($k) . '=' . urlencode(strval($v));
+                return urlencode(strval($k)) . '=' . urlencode(strval($v));
             }, array_keys($data), array_values($data)));
         }
 
diff --git a/tests/PHPCurlClass/PHPCurlClassTest.php b/tests/PHPCurlClass/PHPCurlClassTest.php
index fed7c61..c44cda8 100644
--- a/tests/PHPCurlClass/PHPCurlClassTest.php
+++ b/tests/PHPCurlClass/PHPCurlClassTest.php
@@ -72,6 +72,15 @@ class CurlTest extends \PHPUnit\Framework\TestCase
         }
     }
 
+    public function testBuildPostDataIntegerKey()
+    {
+        $curl = new Curl();
+        $this->assertEquals('abc=foo&123=bar', $curl->buildPostData([
+            'abc' => 'foo',
+            '123' => 'bar',
+        ]));
+    }
+
     public function testUserAgent()
     {
         $php_version = preg_replace('/([\.\+\?\*\(\)\[\]\^\$\/])/', '\\\\\1', 'PHP/' . PHP_VERSION);