Can any Firefox experts help me out with a bookmarklet, please?

A HN comment a while ago reported that you can switch to the 'reader mode' view of a website by prefixing the URL with 'about:reader?url=' - this can be useful for some pages where FF doesn't put the 'reader mode' button in the URL bar.

This seemed like an ideal use for a bookmarklet, so I created one as follows:

   javascript:(function(){
       window.location = "about:reader?url=" + window.location;
   })()

But, this doesn't work. In the console I get the error:

TypeError: Location.href setter: Access to 'about:reader?url=https://www.example.com/' from script denied.

Is there any way to work around this, or do bookmarklets simply not have the permissions to alter the URL like this?