diff --git a/insta-lab/app/card_renderer.py b/insta-lab/app/card_renderer.py
index 5bbfc71..11055d8 100644
--- a/insta-lab/app/card_renderer.py
+++ b/insta-lab/app/card_renderer.py
@@ -6,6 +6,7 @@ import json
 import logging
 import os
 import tempfile
+from pathlib import Path
 from typing import List
 
 from jinja2 import Environment, FileSystemLoader, select_autoescape
@@ -67,6 +68,13 @@ async def render_slate(slate_id: int, template: str = "default/card.html.j2") ->
     if not slate:
         raise ValueError(f"slate {slate_id} not found")
     env = _env()
+
+    # template 파일이 없으면 default로 폴백 (INSTA_DEFAULT_THEME가 import 안 된 theme이면 안전)
+    template_full = Path(_resolve_template_dir()) / template
+    if not template_full.exists():
+        logger.warning("Template '%s' 없음 → 'default/card.html.j2'로 폴백", template)
+        template = "default/card.html.j2"
+
     tmpl = env.get_template(template)
     pages = _build_pages(slate)
     out_dir = _slate_dir(slate_id)
diff --git a/insta-lab/tests/test_card_renderer.py b/insta-lab/tests/test_card_renderer.py
index 647a71d..929a20d 100644
--- a/insta-lab/tests/test_card_renderer.py
+++ b/insta-lab/tests/test_card_renderer.py
@@ -46,3 +46,14 @@ async def test_render_slate_produces_ten_pngs(tmp_db_and_dirs):
     db_module.update_slate_status(sid, "rendered")
     assets = db_module.list_card_assets(sid)
     assert {a["page_index"] for a in assets} == set(range(1, 11))
+
+
+@pytest.mark.asyncio
+async def test_render_falls_back_to_default_when_theme_html_missing(tmp_db_and_dirs):
+    """존재하지 않는 theme HTML 지정 시 default/card.html.j2로 폴백, 정상 PNG 생성."""
+    sid = _seed_slate()
+    paths = await card_renderer.render_slate(sid, template="ghost_theme/card.html.j2")
+    assert len(paths) == 10
+    for p in paths:
+        assert os.path.exists(p)
+        assert os.path.getsize(p) > 1000