getSVGDocument() returns null in Safari

I am having a bit of a problem when trying to debug a webpage in Safari. In my mainpage I've got a iframe which I reload every three seconds to update tags on my mainpage. However, when I try using it in Safari, the following row is null

parent.document.getElementById(tagId).getSVGDocument();

I use this in order to update the fill of SVG objects on my main page. It seems as if the page stops the javascript after this line since it does not continue with the other functions I have on my mainpage.

Does anyone know what might be the problem with getSVGDocument() in Safari? Everything works as it should in IE.

Thanks in advance!

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