diff --git a/btrfs/init.sh b/btrfs/init.sh
index e133874..a6d0fa7 100755
--- a/btrfs/init.sh
+++ b/btrfs/init.sh
@@ -31,7 +31,8 @@ require_root "$0" "$@"
 
 command -v btrfs >/dev/null || { echo "error: btrfs-progs not installed" >&2; exit 1; }
 
-if ! blkid -p -O "$DEV" | grep -q btrfs 2>/dev/null; then
+# Note: `blkid -O` is --offset, not "on device". Use a TYPE tag probe.
+if [[ "$(blkid -p -s TYPE -o value "$DEV" 2>/dev/null)" != "btrfs" ]]; then
   echo "error: $DEV is not a btrfs filesystem" >&2
   exit 1
 fi