nsIDOMCSSStyleSheet.getCssRules()

Hello,

I try to call the methode getCssRules() but it keeps returning an
error.

I call it using queryInterface over every item in
nsIDOMStyleSheetList; here is the code :

	for (int i = 0; i < getStyleSheets().getLength(); i++) {

					nsIDOMStyleSheet domStyleSheet = documentStyle
							.getStyleSheets().item(i);

					 nsIDOMCSSStyleSheet cssStyleSheet = null;

					 try {
					           cssStyleSheet = (nsIDOMCSSStyleSheet)
documentStyle.getStyleSheets()
                                                                            .item
(i)
					                                    .queryInterface(
 
nsIDOMCSSStyleSheet.NS_IDOMCSSSTYLESHEET_IID);

					 } catch (XPCOMException e) {}

					System.out.println("CSSStyleSheet : "
					+ cssStyleSheet.getCssRules().item(i));

				}
          }

but it keeps returning the following error :

The function "cssRules" returned an error condition  (0x805303e8)
 at org.mozilla.xpcom.internal.XPCOMJavaProxy.callXPCOMMethod(Native
Method)

is this the right way to do it or am I missing something?