diff --git a/src/Api/Client.php b/src/Api/Client.php
index 80e83be6..fb2e4a94 100644
--- a/src/Api/Client.php
+++ b/src/Api/Client.php
@@ -349,8 +349,10 @@ protected function _arrayToXml(array $array, SimpleXMLElement $xml, $parentEl =
             $el = is_int($key) && $parentEl ? $parentEl : $key;
             if (is_array($value)) {
                 $this->_arrayToXml($value, $this->_isAssocArray($value) ? $xml->addChild($el) : $xml, $el);
-            } else {
+            } elseif(!isset($xml->{$el})) {
                 $xml->{$el} = (string) $value;
+            } else {
+                $xml->{$el}[] = (string) $value;
             }
         }