App.response and null

Why, if the input field is empty and the user pressed the OK button, the function is not interrupted and the Else block is processed?

app.addSubMenu({ cName: "Test", cParent: "Help", nPos: 0})
app.addMenuItem({ cParent: "Test", cName: "My_button",  nPos: 2, cExec: "NumberX()", cEnable: "event.rc = (event.target != null);"});

function NumberX() 
{
		Msg = app.response(" ", " ");
		if (Msg == null) {
			app.alert("The user did not enter anything and clicked Ok");
			return;}
		else 
		{	
			app.alert("The user entered some number: " + Msg);
		}			
};

app.trustedFunction(NumberX);