Unable to preserve binary transparency with PNG8

Hi,

having an image with binary transparency I am unable to convert it to PNG8
without loosing it.

First I create testing image
# gm convert -size 256x256 xc:green -transparent green -fill blue -draw
"rectangle 66,50 144,150" test.png

then I convert it
# gm convert test.png PNG8:indexed.png
It fails with "gm convert: Invalid background palette index (indexed.png)"

I have tried various -colors values (2, 3, 254, 255, 256), running with
-debug coders reveals warning from libpng

  16:01:00 0:01 0.030u 28295 png.c/PNGWarningHandler/1065/Coder:
      libpng-1.2.50 warning: Invalid background palette index

Am I missing something?

Using ImageMagick 6.8.9-9 Q16, everything works as expected (same commands,
just remove gm from them)

Tested on Debian Wheezy (1.3.16-1.1) and Jessie (1.3.20-3+deb8u1, 1.13.22 -
built from source)

Thanks for any help pointing me in right direction.
Juraj

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