Adding color borders to a PNG file with transparency colors all image backg= round.

I want to add green borders to attached png file, with
command: convert -border 10x10 -bordercolor 'rgb(60,200,0)'
-background 'rgb(255,255,255)' dvorak.png
dvorak-borders.png

Unfortunately, it makes all the image green (because it uses
transparency). It seems, however, that bordercolor should
only affect colors of the border, not background (which I
set to white). 

Is this a bug? Is there a workaround ?

Regards, 
Marcin
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