Tiling image creates black boxes

[kwinz]

I am creating tile from a bigger image.

convert tiling-example.png -crop 256x256 -set filename:tile %[fx:page.y/256]_%[fx:page.x/256] %[filename:tile].png

Source file: https://dl.dropbox.com/u/13281067/tiling-example.png
Result screenshot: https://dl.dropbox.com/u/13281067/Scree ... 9%3A14.png

However the result shows every white tile as black.

I tried to fix this with various combinations of
-background white -flatten
-alpha off
-alpha extract
-alpha On
-colorspace transparent

but none helped. The only workaround that helped was to set -define png:color-type=2

convert tiling-example.png -crop 256x256 -set filename:tile %[fx:page.y/256]_%[fx:page.x/256] -define png:color-type=2 %[filename:tile].png

I am using "ImageMagick 6.7.8-4 2012-07-19 Q16"
Is this a bug? Or am I doning something wrong?
Any help appreciated!

[glennrp]

I believe that bug was introduced by me in IM-6.7.8-3 and fixed in 6.7.8-5

[anthony]

Ensure you quote that argument. [...] are shell meta chars!
But that isn't your problem.

[kwinz]

I still get black boxes in the white area when I view it on my mobile Android phone. On the PC it's ok now.
Version: ImageMagick 6.7.8-6 2012-07-23 Q16