diff --git a/servers/rest_server/includes/RESTServer.inc b/servers/rest_server/includes/RESTServer.inc
index 1c07ca2..ffafa5f 100644
--- a/servers/rest_server/includes/RESTServer.inc
+++ b/servers/rest_server/includes/RESTServer.inc
@@ -341,7 +341,7 @@ class RESTServer {
       // throw a exception if the argument isn't optional.
       if (!isset($arguments[$argument_number])) {
         if (!isset($argument_info['optional']) || !$argument_info['optional']) {
-          return services_error(t('Missing required argument @arg', array('@arg' => $argument_info['name'])), 401);
+          return services_error(t('Missing required argument @arg', array('@arg' => $argument_info['name'])), 400);
         }
         // Set default value or NULL if default value is not set.
         $arguments[$argument_number] = isset($argument_info['default value']) ? $argument_info['default value'] : NULL;