Can't get a property from a XMP sidecar file

I am unable to get a property from an XMP sidecar file. The code snippet below gets the "CreatorTool" property.

The code works if the target file is a JPG, but fails if the target file is a XMP sidecar file.

        if ( !ExternalObject.AdobeXMPScript )

            ExternalObject.AdobeXMPScript = new ExternalObject('lib:AdobeXMPScript');

            targetFile = File.openDialog ("Select test file...", "*.xmp,*.jpg", false)

            xmpFile = new XMPFile(targetFile.fsName, XMPConst.XML, XMPConst.OPEN_FOR_READ);

            xmp = xmpFile.getXMP();

                alert(xmp.getProperty(XMPConst.NS_XMP,"CreatorTool"));

            xmpFile.closeFile(XMPConst.CLOSE_UPDATE_SAFELY);

            unloadXMPLibrary();  

I must be missing something. Any ideas?