Based on advice from this forum, the script now reads: (define (dl-png2transindexed2 file) (let* ( (img (car (file-png-load 1 file file) ) ) (d (car (gimp-image-active-drawable img))) ) (gimp-layer-add-alpha (car (gimp-image-get-active-layer img))) (gimp-by-color-select d '(255 255 255) 0 2 0 0 0 0) (gimp-edit-clear) (gimp-convert-indexed img 0 0 255 0 0 "") (file-png-save 1 img d file file 0 6 0 0 0 1 1) ) ) This yields the following error: Error while executing (dl-png2transindexed2 "/home/dkruger/dev/matlab/sdv4/longislandsaltL10T24.png") ERROR: too few arguments (see errobj) First of all, since it's probably obvious what I'm doing wrong, an answer is always appreciated, but I'd like to know if there is any mini-debugger available, or any way of seeing which line is the cause of the error. The error message says to "see errorobj" but I didn't see anything on the console, or a menu in the fu section that would show me the underlying cause. thanks to all!