jQuery: Javascript throws Error "The operation is insecure" when setting value

I am trying to find a fallback solution for browsers who do not interpret the placeholder attribute for input elements. I have this simple jQuery Script but it throws an Error

SecurityError: "The operation is insecure.
this.value = val;"

Here's my script:

$('document').ready(function(){
       $('input').each(function() {
           if ($(this).val() === '' || $(this).val() === undefined) {
               $(this).val($(this).attr('placeholder'));
           }
       });
});

Anyone any ideas what i can do? Or what i am doing wrong? Or what this error means? It happens in Firefox, haven't tested it in other Browsers yet.