diff --git a/graphgen/models/reader/csv_reader.py b/graphgen/models/reader/csv_reader.py
index 97b26c68..bc865a3b 100644
--- a/graphgen/models/reader/csv_reader.py
+++ b/graphgen/models/reader/csv_reader.py
@@ -6,11 +6,19 @@
 
 
 class CSVReader(BaseReader):
+    """
+    Reader for CSV files.
+    Columns:
+        - type: The type of the document (e.g., "text", "image", etc.)
+        - if type is "text", "content" column must be present.
+    """
+
     def read(self, file_path: str) -> List[Dict[str, Any]]:
 
         df = pd.read_csv(file_path)
         for _, row in df.iterrows():
-            if "type" in row and row["type"] == "text" and self.text_column not in row:
+            assert "type" in row, f"Missing 'type' column in document: {row.to_dict()}"
+            if row["type"] == "text" and self.text_column not in row:
                 raise ValueError(
                     f"Missing '{self.text_column}' in document: {row.to_dict()}"
                 )
diff --git a/graphgen/models/reader/json_reader.py b/graphgen/models/reader/json_reader.py
index 943fbcab..8253041c 100644
--- a/graphgen/models/reader/json_reader.py
+++ b/graphgen/models/reader/json_reader.py
@@ -5,11 +5,19 @@
 
 
 class JSONReader(BaseReader):
+    """
+    Reader for JSON files.
+    Columns:
+        - type: The type of the document (e.g., "text", "image", etc.)
+        - if type is "text", "content" column must be present.
+    """
+
     def read(self, file_path: str) -> List[Dict[str, Any]]:
         with open(file_path, "r", encoding="utf-8") as f:
             data = json.load(f)
             if isinstance(data, list):
                 for doc in data:
+                    assert "type" in doc, f"Missing 'type' in document: {doc}"
                     if doc.get("type") == "text" and self.text_column not in doc:
                         raise ValueError(
                             f"Missing '{self.text_column}' in document: {doc}"
diff --git a/graphgen/models/reader/jsonl_reader.py b/graphgen/models/reader/jsonl_reader.py
index be9f1cca..31bc3195 100644
--- a/graphgen/models/reader/jsonl_reader.py
+++ b/graphgen/models/reader/jsonl_reader.py
@@ -6,12 +6,20 @@
 
 
 class JSONLReader(BaseReader):
+    """
+    Reader for JSONL files.
+    Columns:
+        - type: The type of the document (e.g., "text", "image", etc.)
+        - if type is "text", "content" column must be present.
+    """
+
     def read(self, file_path: str) -> List[Dict[str, Any]]:
         docs = []
         with open(file_path, "r", encoding="utf-8") as f:
             for line in f:
                 try:
                     doc = json.loads(line)
+                    assert "type" in doc, f"Missing 'type' in document: {doc}"
                     if doc.get("type") == "text" and self.text_column not in doc:
                         raise ValueError(
                             f"Missing '{self.text_column}' in document: {doc}"
diff --git a/graphgen/models/reader/parquet_reader.py b/graphgen/models/reader/parquet_reader.py
index 34194a1b..a325b876 100644
--- a/graphgen/models/reader/parquet_reader.py
+++ b/graphgen/models/reader/parquet_reader.py
@@ -8,6 +8,9 @@
 class ParquetReader(BaseReader):
     """
     Read parquet files, requiring the schema to be restored to List[Dict[str, Any]].
+    Columns:
+    - type: The type of the document (e.g., "text", "image", etc.)
+    - if type is "text", "content" column must be present.
     """
 
     def read(self, file_path: str) -> List[Dict[str, Any]]:
@@ -15,6 +18,7 @@ def read(self, file_path: str) -> List[Dict[str, Any]]:
         data: List[Dict[str, Any]] = df.to_dict(orient="records")
 
         for doc in data:
+            assert "type" in doc, f"Missing 'type' in document: {doc}"
             if doc.get("type") == "text" and self.text_column not in doc:
                 raise ValueError(f"Missing '{self.text_column}' in document: {doc}")
         return self.filter(data)
diff --git a/graphgen/models/reader/pickle_reader.py b/graphgen/models/reader/pickle_reader.py
index 96cf8bca..1a11dc11 100644
--- a/graphgen/models/reader/pickle_reader.py
+++ b/graphgen/models/reader/pickle_reader.py
@@ -7,6 +7,10 @@
 class PickleReader(BaseReader):
     """
     Read pickle files, requiring the top-level object to be List[Dict[str, Any]].
+
+    Columns:
+    - type: The type of the document (e.g., "text", "image", etc.)
+    - if type is "text", "content" column must be present.
     """
 
     def read(self, file_path: str) -> List[Dict[str, Any]]:
@@ -19,6 +23,7 @@ def read(self, file_path: str) -> List[Dict[str, Any]]:
         for doc in data:
             if not isinstance(doc, dict):
                 raise ValueError("Every item in the list must be a dict.")
+            assert "type" in doc, f"Missing 'type' in document: {doc}"
             if doc.get("type") == "text" and self.text_column not in doc:
                 raise ValueError(f"Missing '{self.text_column}' in document: {doc}")