我的项目是 sinatra 写的，现在在给网站加验证码，利用 ImageMagick 生成图片

convert -size 100x28 -fill black -background white       -draw 'stroke black line 5,26 62,3'       -draw 'stroke black line 5,0 67,21'       -wave 3x52       -font '/usr/lib/sinatra/public/fonts/SegoePro-Regular.ttf'       -gravity Center -sketch 3x1+134 -pointsize 22 -implode 0.2 label:dsaf png:123.png

结果报错

convert: not authorized `dsaf' @ error/constitute.c/ReadImage/453.
convert: missing an image filename `png:123.png' @ error/convert.c/ConvertImageCommand/3015.

这个怎么解决？