diff --git a/roles/preflight/tasks/main.yml b/roles/preflight/tasks/main.yml
index 4de2e40..53c62f9 100644
--- a/roles/preflight/tasks/main.yml
+++ b/roles/preflight/tasks/main.yml
@@ -24,7 +24,7 @@
     fail_msg: "DISK: {{ item.item }} is {{ item.stdout }}% full — minimum {{ min_free_disk_percent }}% free required"
     success_msg: "Disk OK: {{ item.item }} is {{ item.stdout }}% used"
   loop: "{{ disk_usage.results }}"
-  when: item.rc == 0
+  when: item.rc == 0 and item.stdout | int(-1) >= 0  # skip if stdout is not numeric (e.g. /boot missing on Alpine LXC)
 
 - name: Get load average
   ansible.builtin.shell: |