See:
https://notabug.org/mcz/spm/commit/8c3120fd44df117e5947d2705ba0d87443be831c.diff
--- spm.orig	2017-10-22 12:27:00 UTC
+++ spm
@@ -21,6 +21,7 @@ umask u=rwx,go=
 ## Variables
 GPG_OPTS='--quiet --yes --batch'
 STORE_DIR="${PASSWORD_STORE_DIR:-${HOME}/.spm}"
+STORE_KEY="${PASSWORD_STORE_KEY:-}"
 
 ## Helper
 usage() {
@@ -41,10 +42,10 @@ check() {
 }
 
 gpg() {
-	if [ -z "${PASSWORD_STORE_KEY}" ]; then
+	if [ -z "${STORE_KEY}" ]; then
 		gpg2 ${GPG_OPTS} --default-recipient-self "${@}"
 	else
-		gpg2 ${GPG_OPTS} --recipient "${PASSWORD_STORE_KEY}" "${@}"
+		gpg2 ${GPG_OPTS} --recipient "${STORE_KEY}" "${@}"
 	fi
 }
 
@@ -53,6 +54,7 @@ readpw() {
 	IFS= read -r "${2}"
 	[ -t 0 ] && stty echo
 	[ -z "${2}" ] && usage 'empty password'
+	return 0
 }
 
 find() {