diff --git a/data/helpers.d/filesystem b/data/helpers.d/filesystem
index f8da72ad5..8f6665891 100644
--- a/data/helpers.d/filesystem
+++ b/data/helpers.d/filesystem
@@ -46,10 +46,11 @@ ynh_backup() {
     local SRC_PATH="$1"
     local DEST_PATH="${2:-}"
     local IS_BIG="${3:-0}"
+    BACKUP_CORE_ONLY=${BACKUP_CORE_ONLY:-0}
 
     # If backing up core only (used by ynh_backup_before_upgrade),
     # don't backup big data items
-    if [ "$IS_BIG" == "1" ] && [ -n "$BACKUP_CORE_ONLY" ] ; then
+    if [ "$IS_BIG" == "1" ] && [ "$BACKUP_CORE_ONLY" == "1" ] ; then
       echo "$SRC_PATH will not be saved, because backup_core_only is set." >&2
       return 0
     fi