Safari extension removeContentStyleSheet(url) removes all extension content style sheets

In a Safari extension, when calling safari.extension.removeContentStyleSheet(url)

• if url is an existing content style sheet (added in Info.plist OR programmatically with addContentStyleSheetFromURL), all content style sheets from the extension are removed, not just the one referred to by url
• if url is not an existing style sheet, behaviour is as expected (as in nothing happens)

This is obviously a major problem. safari.extension.removeContentStyleSheet should selectively remove individual content style sheets. If I wanted to remove all of them, there is already safari.extension.removeContentStyleSheets

Currently using Safari 5.1.1 (7534.51.22) on Windows XP SP3.