diff --git a/website/scripts/vercel-ignore.sh b/website/scripts/vercel-ignore.sh
index 756a15a4..f5d0aa9d 100755
--- a/website/scripts/vercel-ignore.sh
+++ b/website/scripts/vercel-ignore.sh
@@ -1,11 +1,24 @@
 #!/usr/bin/env bash
 # Vercel "Ignored Build Step" (see website/vercel.json).
-# Exit 0 = skip the build, exit 1 = build. Vercel's working directory is website/.
+# Exit 0 = skip the build, exit 1 = build. Any other exit code makes Vercel
+# fail the deployment, so every error path below must end in exit 1 (build).
+# Vercel's working directory is website/.
 # - Production always builds.
 # - A preview whose commit message contains "[vercel skip]" is skipped (deploy budget).
 # - Otherwise build only when website/, docs/ or CHANGELOG.md changed.
+# - If the previous deployed commit is unavailable (e.g. the branch was
+#   force-pushed, or it is outside Vercel's shallow clone), build.
 if [ "$VERCEL_ENV" = "production" ]; then exit 1; fi
 case "$VERCEL_GIT_COMMIT_MESSAGE" in
   *'[vercel skip]'*) echo 'Preview skipped: [vercel skip] in commit message'; exit 0 ;;
 esac
-git diff --quiet "${VERCEL_GIT_PREVIOUS_SHA:-HEAD^}" HEAD -- . ../docs ../CHANGELOG.md
+prev="${VERCEL_GIT_PREVIOUS_SHA:-HEAD^}"
+if ! git cat-file -e "${prev}^{commit}" 2>/dev/null; then
+  echo "Previous commit ${prev} is not available; building"
+  exit 1
+fi
+if git diff --quiet "$prev" HEAD -- . ../docs ../CHANGELOG.md; then
+  echo 'Preview skipped: no changes under website/, docs/ or CHANGELOG.md'
+  exit 0
+fi
+exit 1