diff --git a/src/Api/MailingList/Member.php b/src/Api/MailingList/Member.php
index 27068e6..eb99ae0 100644
--- a/src/Api/MailingList/Member.php
+++ b/src/Api/MailingList/Member.php
@@ -152,7 +152,6 @@ class Member extends HttpApi
                     case 'vars':
                         if (is_array($value)) {
                             $value = json_encode($value);
-                            $parameters[$field] = $value;
                         }
                         // We should assert that "vars"'s $value is a string.
                         // no break
@@ -195,12 +194,11 @@ class Member extends HttpApi
         Assert::stringNotEmpty($address);
         Assert::isArray($parameters);
 
-        foreach ($parameters as $field => $value) {
+        foreach ($parameters as $field => &$value) {
             switch ($field) {
                 case 'vars':
                     if (is_array($value)) {
                         $value = json_encode($value);
-                        $parameters[$field] = $value;
                     }
                     // We should assert that "vars"'s $value is a string.
                     // no break