diff --git a/base-image-script/bootstrap-rootfs.sh b/base-image-script/bootstrap-rootfs.sh
index 11a251e..29d087a 100644
--- a/base-image-script/bootstrap-rootfs.sh
+++ b/base-image-script/bootstrap-rootfs.sh
@@ -2,9 +2,9 @@
 set -euo pipefail
 
 DEBIAN_RELEASE="${VERSION:-}"
-ROOTFS_DIR="debian-${DEBIAN_RELEASE}"
+ROOTFS_DIR="debian-$DEBIAN_RELEASE"
 OUTPUT_DIR="/output"
-TARBALL="${OUTPUT_DIR}/debian-${DEBIAN_RELEASE}.tar.bz2"
+TARBALL="$OUTPUT_DIR/debian-$DEBIAN_RELEASE.tar.bz2"
 
 echo "[INFO] Bootstrapping Debian $DEBIAN_RELEASE rootfs..."
 apt-get update