"Empty input file" error for image loaded from url

[Wolfram Müller]

I have a loop that runs through a list of image-urls and opens the url with

Miro::DominantColors.new('http://www.example.com/image.jpg')

for some images it returns:

convert: Empty input file `/tmp/source20120709-6914-7hzuos.jpg' @ error/jpeg.c/EmitMessage/236.
convert: missing an image filename `/tmp/downsampled20120709-6914-1ldgk69.png' @ error/convert.c/ConvertImageCommand/3011.
/home/wolle/.rvm/gems/ruby-1.9.3-p194/gems/cocaine-0.2.1/lib/cocaine/command_line.rb:39:in `run': Command '/usr/bin/convert '/tmp/source20120709-6914-7hzuos.jpg' -resize '150x150' -colors '8' '/tmp/downsampled20120709-6914-1ldgk69.png'' returned 1. Expected 0 (Cocaine::ExitStatusError)
    from /home/wolle/.rvm/gems/ruby-1.9.3-p194/gems/miro-0.2.1/lib/miro/dominant_colors.rb:46:in `downsample_colors_and_convert_to_png!'
    from /home/wolle/.rvm/gems/ruby-1.9.3-p194/gems/miro-0.2.1/lib/miro/dominant_colors.rb:33:in `extract_colors_from_image'
    from /home/wolle/.rvm/gems/ruby-1.9.3-p194/gems/miro-0.2.1/lib/miro/dominant_colors.rb:28:in `sorted_pixels'
    from /home/wolle/.rvm/gems/ruby-1.9.m3-p194/gems/miro-0.2.1/lib/miro/dominant_colors.rb:10:in `to_hex'
    from color.rb:23:in `get_main_color'
    from color.rb:55:in `block in start'
    from color.rb:50:in `each'
    from color.rb:50:in `start'
    from color.rb:19:in `initialize'
    from color.rb:66:in `new'
    from color.rb:66:in `<main>

i tripple checked. all the image-urls are correct. what else could be the problem?