UIComponent as Button icon style causes "ArgumentError: undefined state 'up'".

Steps to reproduce:
1. Create a subclass of UIComponent to act as a button icon. The one I use does nothing but set its measuredWidth/Height to 16 X 16 and paint pixels in the updateDisplayList method.
2. Use this class as the "icon" style in a Button

Actual Results:
Somewhere, some code is trying to set the "currentState" property of the icon instance to "up", (which is obviously a button state) but this causes an error because "up" is not a valid state for the icon. The error message is:

ArgumentError: Undefined state 'up'.
at mx.core::UIComponent/::getState()
at mx.core::UIComponent/::findCommonBaseState()
at mx.core::UIComponent/::commitCurrentState()
at mx.core::UIComponent/setCurrentState()
at mx.core::UIComponent/set currentState()
at mx.controls::Button/http://www.adobe.com/2006/flex/mx/internal::viewIconForPhase()
at mx.controls::Button/http://www.adobe.com/2006/flex/mx/internal::viewIcon()
at mx.controls::Button/mx.controls:Button::commitProperties()
at mx.core::UIComponent/validateProperties()
at mx.managers::LayoutManager/::validateProperties()

Expected Results:
It should work without throwing this error, as it does in Flex 2.0.1. In Flex 2, the currentState setter of the icon object is not called.

Workaround (if any):
a) Use an embedded image as the icon.
b) Extend mx.skins.ProgrammaticSkin instead of UIComponent (see workaround.zip for example)