RMagick error using script/server but works in unit test

ruby-talk
1

Hello,

We have a method which converts a PDF from the filesystem to a
scaled JPEG data stream:

   def lowres_jpeg_data
     image = Magick::ImageList.new self.class.base_path + "/#{id}.pdf"
     image.scale!(1000,1000)
     image.format = 'JPEG'
     image.to_blob
   end

The unit test passes:

   def test_lowres_jpeg_data
     expected = File.read RAILS_ROOT + '/test/fixtures/images/scales_and_crops/1_lowres.jpg'
     image = Image.find 1
     assert_equal expected, image.lowres_jpeg_data
   end

However, when the method is called from the application running under
the webrick server in development mode we hit the following exception
using the same PDF file.

Magick::ImageMagickError (Postscript delegate failed `./var/cache/1.pdf': ):

How can this work in a test but then not in the application?

OS X server 10.4.3
rmagick (1.9.3)
rails (0.14.2)
AFPL Ghostscript 8.51 (2005-04-18)
ImageMagick graphics/ImageMagick 6.2.4-3