diff --git a/trl/trainer/bco_trainer.py b/trl/trainer/bco_trainer.py
index 967373713d..67733a48a6 100644
--- a/trl/trainer/bco_trainer.py
+++ b/trl/trainer/bco_trainer.py
@@ -105,7 +105,9 @@ def _tokenize(
     full_input_ids = [np.array(f) for f in full_input_ids]
     for full, concat in zip(full_input_ids, full_concat_input_ids):
         if len(full) != len(concat):
-            raise ValueError("Prompt input ids and answer input ids should have the same length.")
+            raise ValueError(
+                "The elements in 'full_input_ids' and 'full_concat_input_ids' must have the same pairwise length."
+            )
 
     # On some tokenizers, like Llama-2 tokenizer, there are occasions where tokens
     # can be merged together when tokenizing prompt+answer. This could result
diff --git a/trl/trainer/kto_trainer.py b/trl/trainer/kto_trainer.py
index 7be86d924e..17ad779a5a 100644
--- a/trl/trainer/kto_trainer.py
+++ b/trl/trainer/kto_trainer.py
@@ -110,7 +110,9 @@ def _tokenize(
     full_input_ids = [np.array(f) for f in full_input_ids]
     for full, concat in zip(full_input_ids, full_concat_input_ids):
         if len(full) != len(concat):
-            raise ValueError("Prompt input ids and answer input ids should have the same length.")
+            raise ValueError(
+                "The elements in 'full_input_ids' and 'full_concat_input_ids' must have the same pairwise length."
+            )
 
     # On some tokenizers, like Llama-2 tokenizer, there are occasions where tokens
     # can be merged together when tokenizing prompt+answer. This could result