diff --git a/llm_web_kit/model/lang_id.py b/llm_web_kit/model/lang_id.py
index 92121af7..03945638 100644
--- a/llm_web_kit/model/lang_id.py
+++ b/llm_web_kit/model/lang_id.py
@@ -234,7 +234,7 @@ def decide_language_func(content_str: str, lang_detect: LanguageIdentification)
 
     # return "empty" if the content string is empty
     if len(content_str.strip()) == 0:
-        return {'language': 'empty', 'language_details': None}
+        return {'language': 'empty', 'language_details': 'empty'}
 
     if lang_detect.version not in LANG_ID_SUPPORTED_VERSIONS:
         raise ValueError(f'Unsupported version: {lang_detect.version}. Supported versions: {LANG_ID_SUPPORTED_VERSIONS}')
@@ -242,12 +242,13 @@ def decide_language_func(content_str: str, lang_detect: LanguageIdentification)
     predictions, probabilities = lang_detect.predict(content_str)
     language = decide_language_by_prob_v176(predictions, probabilities)
 
-    language_details = None
     if lang_detect.version == '218.bin':
         first_pred = predictions[0]
         # Extract the full label (e.g., __label__eng_Latn -> eng_Latn)
         if first_pred.startswith('__label__'):
             language_details = first_pred.replace('__label__', '')
+    else:
+        language_details = 'not_defined'
 
     return {
         'language': language,
diff --git a/tests/llm_web_kit/model/test_lang_id.py b/tests/llm_web_kit/model/test_lang_id.py
index 2c1b1f96..5e1a3023 100644
--- a/tests/llm_web_kit/model/test_lang_id.py
+++ b/tests/llm_web_kit/model/test_lang_id.py
@@ -125,7 +125,7 @@ def test_decide_language_func():
     lang_detect.version = '176.bin'
     lang_detect.predict.return_value = (['__label__en', '__label__zh'], [0.6, 0.4])
     result = decide_language_func('test text', lang_detect)
-    assert result == {'language': 'en', 'language_details': None}
+    assert result == {'language': 'en', 'language_details': 'not_defined'}
 
     # Test for 218.bin version
     lang_detect.version = '218.bin'
@@ -135,7 +135,7 @@ def test_decide_language_func():
 
     # Test for empty string
     result = decide_language_func('', lang_detect)
-    assert result == {'language': 'empty', 'language_details': None}
+    assert result == {'language': 'empty', 'language_details': 'empty'}
 
 
 def test_update_language_by_str():