diff --git a/src/Exception/Evaluation.php b/src/Exception/Evaluation.php
index 73669a2..4871c2a 100644
--- a/src/Exception/Evaluation.php
+++ b/src/Exception/Evaluation.php
@@ -44,7 +44,8 @@ public function __construct($message = '', $code = 0)
 
     private static function isValidCode($code)
     {
-        return is_int($code) && (($code < -32768) || (-32000 < $code));
+        // as defined by the JSON-RPC 2.0 spec, see http://www.jsonrpc.org/specification#error_object
+        return is_int($code) && (-32768 <= $code && $code <= -32000);
     }
 
     private static function isValidMessage($message)