applyLeafFormat not working if span text is empty

Hi - We have a text area, where we can type in text, change font, size etc.  The change font or color uses applyLeafFormat. It works fine if there is some text. If the span text is empty, change font or size does not work. here is what am doing for change font size. Pls guide me whats wrong and why is it not working if there is no text .

                var tlf:TextLayoutFormat = new TextLayoutFormat();
                tlf.fontSize = int(newFontSize);
                var editManager:IEditManager = textFlow.interactionManager as IEditManager;
                textFlow.interactionManager.selectAll();
                editManager.applyLeafFormat(tlf);
                textFlow.interactionManager.selectRange(textFlow.interactionManager.absoluteEnd, textFlow.interactionManager.absoluteEnd);
                textFlow.interactionManager.setFocus();