convert erroneously returns blank image

I'm splitting an image into two pieces with this:

convert -crop 1x2+150+150@ +repage +adjoin img.png img-%d.png

Then I rotate each piece like so:

convert -rotate -90 img-0.png img-0-rotated.png
convert -rotate -90 img-1.png img-1-rotated.png

Then I convert each rotated image to a PDF like so:

convert img-0-rotated.png img-0-final.pdf
convert img-1-rotated.png img-1-final.pdf

img-0-final.pdf turns out as expected, but for some reason,
img-1-final.pdf always ends up blank. Both rotated images look right,
so the bug is in the conversion from png to pdf. Any ideas what I
might be doing wrong here?

Thanks,
Carl