diff --git a/src/yomitoku/export/export_csv.py b/src/yomitoku/export/export_csv.py
index 48cd7dd..2247db6 100644
--- a/src/yomitoku/export/export_csv.py
+++ b/src/yomitoku/export/export_csv.py
@@ -41,6 +41,8 @@ def save_figure(
     out_path,
     figure_dir="figures",
 ):
+    assert img is not None, "img is required for saving figures"
+
     for i, figure in enumerate(figures):
         x1, y1, x2, y2 = map(int, figure.box)
         figure_img = img[y1:y2, x1:x2, :]
diff --git a/src/yomitoku/export/export_html.py b/src/yomitoku/export/export_html.py
index 3670f96..180b975 100644
--- a/src/yomitoku/export/export_html.py
+++ b/src/yomitoku/export/export_html.py
@@ -110,6 +110,8 @@ def figure_to_html(
     figure_dir="figures",
     width=200,
 ):
+    assert img is not None, "img is required for saving figures"
+
     elements = []
     for i, figure in enumerate(figures):
         x1, y1, x2, y2 = map(int, figure.box)
diff --git a/src/yomitoku/export/export_json.py b/src/yomitoku/export/export_json.py
index 819ea05..3b41c2a 100644
--- a/src/yomitoku/export/export_json.py
+++ b/src/yomitoku/export/export_json.py
@@ -21,6 +21,8 @@ def save_figure(
     out_path,
     figure_dir="figures",
 ):
+    assert img is not None, "img is required for saving figures"
+
     for i, figure in enumerate(figures):
         x1, y1, x2, y2 = map(int, figure.box)
         figure_img = img[y1:y2, x1:x2, :]
diff --git a/src/yomitoku/export/export_markdown.py b/src/yomitoku/export/export_markdown.py
index da9c54e..ebf5811 100644
--- a/src/yomitoku/export/export_markdown.py
+++ b/src/yomitoku/export/export_markdown.py
@@ -75,6 +75,8 @@ def figure_to_md(
     width=200,
     figure_dir="figures",
 ):
+    assert img is not None, "img is required for saving figures"
+
     elements = []
     for i, figure in enumerate(figures):
         x1, y1, x2, y2 = map(int, figure.box)
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 9be86b9..2a49795 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -210,6 +210,7 @@ def test_validate_encoding():
         validate_encoding("utf-9")
 
     assert validate_encoding("utf-8")
+    assert validate_encoding("utf-8-sig")
     assert validate_encoding("shift-jis")
     assert validate_encoding("euc-jp")
     assert validate_encoding("cp932")