Use just the car of what find-operation-coding-system returns.
(let ((buffer-read-only nil)
(coding-system-for-write 'no-conversion)
(coding-system-for-read
- (find-operation-coding-system
- 'insert-file-contents
- buffer-file-name t)))
+ (car (find-operation-coding-system
+ 'insert-file-contents
+ buffer-file-name t))))
(shell-command-on-region (point-min) (point-max) uncompress-program t))
(goto-char (point-min))
(message "Uncompressing...done")