Hi just this morning I encountered the same problem, I'm still on rails 3.2 but this one worked fine for me. I guess its something with your yield function.
process :resize_to_limit => [800, 800]
process :add_text
def add_text
manipulate! do |image|
image.combine_options do |c|
c.gravity 'Center'
c.pointsize '22'
c.draw "text 0,0 'test'"
c.fill 'white'
end
image
end
end