diff --git a/pkg/de-p1st-installer/lib/block-device.sh b/pkg/de-p1st-installer/lib/block-device.sh
index b6d8f28..6a91199 100644
--- a/pkg/de-p1st-installer/lib/block-device.sh
+++ b/pkg/de-p1st-installer/lib/block-device.sh
@@ -56,7 +56,7 @@ function partition() {
   done
 
   # if BOOT_PART_SIZE not given, set to default value
-  "${BOOT_PART_SIZE:='261'}"
+  BOOT_PART_SIZE="${BOOT_PART_SIZE:='261'}"
   # if too small, print warning and exit
   if [ "${BOOT_PART_SIZE}" -lt "261" ]; then
     echo "BOOT_PART_SIZE should be larger than 260!";