Javascript – Can’t set parent hidden input from iframe!

I have an iframe on my main page, and this iframe uses php code mostly (src is php file).

At the bottom of this iframe I have this:

   window.parent.document.getElementById("qry_str").value='hey';

And in the main (parent) file, I have this hidden input which I am trying to set from the iframe with js:

         <input type="hidden" id="qry_str" name="qry_str">

Nothing happens when I try to set the value of this hidden input from inside the iframe.
(Nothing shows up in the src code that is)!
I have also tried setting the hidden inputs 'OnChange' event to an alert, but it won't alert because it isn't changed I guess!

I know it finds the element because I have tried to alert the elementId and it works, the element is found!

Thankful for any help!