diff --git a/lib/MongoLite/Database.php b/lib/MongoLite/Database.php
index 968b4484e..2ee88bebc 100644
--- a/lib/MongoLite/Database.php
+++ b/lib/MongoLite/Database.php
@@ -290,6 +290,10 @@ public static function buildCondition($criteria, $concat = ' && ') {
 
                     $d = '$document';
 
+                    if (\strpos($key, '(') !== false) {
+                        throw new \InvalidArgumentException('Unallowed characters used in filter keys');
+                    }
+
                     if (\strpos($key, '.') !== false) {
 
                         $keys = \explode('.', $key);