[SOLVED] lossless jpeg join, crop'n'drop

I am trying to use this:
https://stackoverflow.com/questions/...15714#29615714

to join two jpeg files without recoding.

but, whenever I try to 'crop' image to larger size:

jpegtran -crop 512x256+0+0 -outfile O.jpg A.jpg

it says:
"Invalid crop request"
?

[using jpegtran from debian stable: libjpeg-turbo-progs (1:1.5.2-2 and others)]