diff --git a/scripts/build-installer b/scripts/build-installer
index f989d01..10cd84c 100755
--- a/scripts/build-installer
+++ b/scripts/build-installer
@@ -10,7 +10,8 @@ if [ ! -d /gnu/store ]; then
 # servers for nonguix and sakura
 guix archive --authorize < "$repo/files/keys/nonguix.pub"
 guix archive --authorize < "$repo/files/keys/sakura.pub"
-substitutes=$((cat <<EOF
+substitutes=$(
+(cat <<EOF
 https://substitutes.nonguix.org
 https://substitutes.freya.cat
 https://bordeaux.guix.gnu.org