EMPTY_SPARSE_EXTENDED was shared across lines via copyFrom; addon-image writes _extendedAttrs directly without copy-on-write, which caused tile eviction failures and broke instanceof Object checks. Use undefined for empty extended maps (keep EMPTY_SPARSE_MAP for _combined). Ensure addon-image forks the map before writes and update the accessor test. Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>