Bug or malformed PNG?

PngImageEncoder in org.apache.batik.ext.awt.image.codec.png is dumping my
attempts to make a SVG file with a RuntimeException (line 886 : public void
encode(RenderedImage im))

A quick dig through the code showed it does this when the channels in a PNG
did not have an equal number of bits -->

            this.bitDepth = sampleSize[0];
            // Ensure all channels have the same bit depth
            for (int i = 1; i < sampleSize.length; i++) {
                if (sampleSize[i] != bitDepth) {
                    throw new RuntimeException();
                }
            }


and interrogating my PNG files showed this was true for a couple of them. I
have a couple of PNGs with 8-bit red/green/blue channels and a 1-bit alpha
channel, equating to what is a transparent bitmasked image (rather than
translucent)

So my question is has my paint program (Paint.NET on Windows 7) generated a
bad PNG, or is this a bug in batik?

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