diff --git a/gemma/gm/data/_functional.py b/gemma/gm/data/_functional.py
index b9e4fef5..d1bbfc47 100644
--- a/gemma/gm/data/_functional.py
+++ b/gemma/gm/data/_functional.py
@@ -14,6 +14,8 @@
 
 """Functional version of the `gm.data` transforms."""
 
+import warnings
+
 from etils import enp
 import flax
 import jax
@@ -134,6 +136,17 @@ def make_seq2seq_fields(
   Returns:
     The input, target and mask, all of length `prompt_len + response_len - 1`.
   """
+  # Handle empty prompt: issue warning and use a default BOS token (2) as fallback.
+  if len(prompt) == 0:
+    warnings.warn(
+        'Empty prompt provided. Using default BOS token (2) as prompt. '
+        'Empty prompts are not recommended for sequence-to-sequence training.',
+        UserWarning,
+        stacklevel=2,
+    )
+    # Use BOS token (2) as a default prompt token for seq2seq compatibility.
+    prompt = np.array([2], dtype=np.int32)
+  
   # Concatenate the prompt and response tokens.
   sequence = np.concatenate([prompt, response])