diff --git a/.gitea/workflows/cd.yml b/.gitea/workflows/cd.yml
index 5cf6d5c..1bf9c3c 100644
--- a/.gitea/workflows/cd.yml
+++ b/.gitea/workflows/cd.yml
@@ -45,6 +45,12 @@ jobs:
           set -e
           trap 'rm -rf /tmp/infra-update; rm -f ~/.ssh/infra_deploy_key' EXIT
           IMAGE_TAG="${{ github.sha }}"
+          # infra#311: github.sha is never empty in practice, but the manifest
+          # patch below is the same shape that took Flux down estate-wide on
+          # 2026-08-25 -- an empty tag yields `image: <repo>:`, which is invalid
+          # YAML and fails the ENTIRE Kustomization, not just this app. The
+          # assert costs nothing and removes the class.
+          [ -n "${IMAGE_TAG:-}" ] || { echo "IMAGE_TAG is empty - refusing to patch"; exit 1; }
           mkdir -p ~/.ssh
           echo "${{ secrets.INFRA_DEPLOY_KEY }}" > ~/.ssh/infra_deploy_key
           chmod 600 ~/.ssh/infra_deploy_key