diff --git a/tools/services/check_service.py b/tools/services/check_service.py
index 442d840..51ed4d0 100644
--- a/tools/services/check_service.py
+++ b/tools/services/check_service.py
@@ -23,7 +23,7 @@ def check_glyph_files(font_config: FontConfig):
         width_mode_dir = path_define.patch_glyphs_dir.joinpath(str(font_config.font_size), width_mode_dir_name)
         context = glyph_file_util.load_context(width_mode_dir)
         for code_point, flavor_group in context.items():
-            assert '' in flavor_group, f'[{font_config.font_size}px] missing default flavor: {width_mode_dir_name} {code_point:04X}'
+            assert None in flavor_group, f'[{font_config.font_size}px] missing default flavor: {width_mode_dir_name} {code_point:04X}'
 
             if code_point == -1:
                 block = None
diff --git a/tools/services/font_service.py b/tools/services/font_service.py
index cb101a7..7d00f92 100644
--- a/tools/services/font_service.py
+++ b/tools/services/font_service.py
@@ -33,10 +33,10 @@ def load(font_config: FontConfig) -> 'DesignContext':
                     flavor_group['zh_hans'] = flavor_group['zh_cn']
                 if 'zh_tr' in flavor_group:
                     flavor_group['zh_hant'] = flavor_group['zh_tr']
-                if '' not in flavor_group:
+                if None not in flavor_group:
                     for language_flavor in configs.language_file_flavors:
                         if language_flavor in flavor_group:
-                            flavor_group[''] = flavor_group[language_flavor]
+                            flavor_group[None] = flavor_group[language_flavor]
                             break
 
         return DesignContext(font_config, glyph_files)