Index: lib/content/elemHideEmulation.js
===================================================================
--- a/lib/content/elemHideEmulation.js
+++ b/lib/content/elemHideEmulation.js
@@ -182,17 +182,17 @@
  * Make a regular expression from a text argument. If it can be parsed as a
  * regular expression, parse it and the flags.
  * @param {string} text the text argument.
  * @return {?RegExp} a RegExp object or null in case of error.
  */
 function makeRegExpParameter(text)
 {
   let [, pattern, flags] =
-      regexpRegexp.exec(text) || [undefined, textToRegExp(text)];
+      regexpRegexp.exec(text) || [null, textToRegExp(text)];
 
   try
   {
     return new RegExp(pattern, flags);
   }
   catch (e)
   {
   }