diff --git a/includes/bootstrap.inc b/includes/bootstrap.inc
index 37348de92229928057bb90746ee856c9da0326c8..ed301c8a76bb4e7be3bb814451301216dceccfab 100644
--- a/includes/bootstrap.inc
+++ b/includes/bootstrap.inc
@@ -1822,7 +1822,8 @@ function t($string, array $args = array(), array $options = array()) {
 function format_string($string, array $args = array()) {
   // Transform arguments before inserting them.
   foreach ($args as $key => $value) {
-    switch ($key[0]) {
+    $type = is_string($key) ? $key[0] : '!';
+    switch ($type) {
       case '@':
         // Escaped only.
         $args[$key] = check_plain($value);