diff --git a/nanochat/dataloader.py b/nanochat/dataloader.py
index 4cb2279..7f3c3d1 100644
--- a/nanochat/dataloader.py
+++ b/nanochat/dataloader.py
@@ -35,6 +35,17 @@ def _document_batches(split, resume_state_dict, tokenizer_batch_size):
     warn_on_legacy = ddp_rank == 0 and split == "train" # rank 0 on train split will warn on legacy
     parquet_paths = list_parquet_files(warn_on_legacy=warn_on_legacy)
     assert len(parquet_paths) != 0, "No dataset parquet files found, did you run dataset.py?"
+
+    # Split parquet files: last file for validation, rest for training
+    # Handle edge case: single file scenario
+    if len(parquet_paths) == 1:
+        import warnings
+        warnings.warn(
+            "Only 1 parquet file found. "
+            "Both train and val will use the same data, which may cause overfitting. "
+            "Consider splitting your dataset into multiple parquet files.",
+            UserWarning
+        )
     parquet_paths = parquet_paths[:-1] if split == "train" else parquet_paths[-1:]
 
     resume_pq_idx = resume_state_dict["pq_idx"] if resume_state_dict is not None else 0