diff --git a/lib/sig.js b/lib/sig.js
index dad9de5b..8de9948f 100644
--- a/lib/sig.js
+++ b/lib/sig.js
@@ -109,7 +109,7 @@ var spliceStr = ':function\\(a,b\\)\\{' +
   'a\\.splice\\(0,b\\)' +
 '\\}';
 var swapStr = ':function\\(a,b\\)\\{' +
-  'var c=a\\[0\\];a\\[0\\]=a\\[b%a\\.length\\];a\\[b\\]=c(?:;return a)?' +
+  'var c=a\\[0\\];a\\[0\\]=a\\[b(?:%a\\.length)?\\];a\\[b(?:%a\\.length)?\\]=c(?:;return a)?' +
 '\\}';
 var actionsObjRegexp = new RegExp(
   `var (${jsVarStr})=\\{((?:(?:` +