$ cat test.sh
:
while getopts d: opt; do
        case "$opt" in
                d) dir="$(readlink -ev "${OPTARG:?}")" ;;
                *) exit 2 ;;
        esac
done
$ bash -e test.sh -d ''
test.sh: line 1: OPTARG: parameter null or not set
$ ash -e test.sh -d '' 
test.sh: 7: OPTARG: parameter not set or null
$ 

Why does bash says it was line 1?
I guess when bash enters subshell, internal line counter is reset to 1.

And why does ash says it was line 7?
It must have been line 4!