How to reset format to e.g. fontWeight==undefined?

I want to reset formats to the default value "undefined" but only get "inherit". Aren't those values for intheritable format properties equal in the TextFlow model?

    public function test_reset_format():void 
    {
        var init:XML = <TextFlow xmlns="http://ns.adobe.com/textLayout/2008"><p><span>aaa</span></p></TextFlow>
        var textFlow:TextFlow = TextConverter.importToFlow(init, TextConverter.TEXT_LAYOUT_FORMAT)
        var editManager:IEditManager = new EditManager()
        textFlow.interactionManager = editManager
        
         var format:TextLayoutFormat = new TextLayoutFormat()
        format.fontWeight = FontWeight.BOLD
        
        editManager.selectRange(0,1)
        editManager.applyLeafFormat(format)
        assertEquals(FontWeight.BOLD, textFlow.getFirstLeaf().fontWeight)
    
         var resetFormat:TextLayoutFormat = new TextLayoutFormat()
        resetFormat.fontWeight = FormatValue.INHERIT
        editManager.applyLeafFormat(resetFormat)
        
        var resetFontWeight:* = textFlow.getFirstLeaf().fontWeight
        assertEquals(FormatValue.INHERIT, resetFontWeight)
        // How can I reset the format so that assertTrue(undefined == resetFontWeight) passes?
    }

Thanks for any hints,

Marc