diff --git a/tests/unit/utilities/test_get_input_with_manually_prepended_bos.py b/tests/unit/utilities/test_get_input_with_manually_prepended_bos.py
new file mode 100644
index 000000000..f11a38bac
--- /dev/null
+++ b/tests/unit/utilities/test_get_input_with_manually_prepended_bos.py
@@ -0,0 +1,70 @@
+"""Tests for get_input_with_manually_prepended_bos when the tokenizer has no BOS token.
+
+``to_tokens`` reaches this helper whenever the caller wants a BOS that the tokenizer
+will not add on its own — ``prepend_bos and not cfg.tokenizer_prepends_bos``. For a
+tokenizer with no BOS token that condition is *correctly* true rather than stale:
+``detect_tokenizer_bos_eos`` requires a ``bos_token_id``, so it reports False for
+BERT and T5, and ``prepend_bos`` defaults to True. The helper then evaluated
+``None + input`` and raised ``TypeError: unsupported operand type(s) for +:
+'NoneType' and 'str'``, naming neither the tokenizer nor the flag.
+
+This is the prepend-side counterpart of #1628, which covers the removal side.
+"""
+
+from __future__ import annotations
+
+import pytest
+from transformers import AutoTokenizer
+
+from transformer_lens.utilities.tokenize_utils import (
+    get_input_with_manually_prepended_bos,
+)
+
+
+@pytest.fixture(
+    scope="module",
+    params=["google-bert/bert-base-cased", "google-t5/t5-small"],
+)
+def no_bos_tokenizer(request):
+    """BERT opens with [CLS] and T5 with nothing, so bos_token is None for both."""
+    tokenizer = AutoTokenizer.from_pretrained(request.param)
+    assert tokenizer.bos_token is None
+    return tokenizer
+
+
+@pytest.fixture(scope="module")
+def bos_tokenizer():
+    tokenizer = AutoTokenizer.from_pretrained("distilgpt2")
+    assert tokenizer.bos_token is not None
+    return tokenizer
+
+
+def test_no_bos_token_returns_string_unchanged(no_bos_tokenizer) -> None:
+    """There is no BOS to prepend, so the string must come back untouched."""
+    assert get_input_with_manually_prepended_bos(no_bos_tokenizer.bos_token, "hello world") == (
+        "hello world"
+    )
+
+
+def test_no_bos_token_returns_list_unchanged(no_bos_tokenizer) -> None:
+    """Same for the batched form — and no partially-prepended list."""
+    inputs = ["hello world", "second string"]
+
+    result = get_input_with_manually_prepended_bos(no_bos_tokenizer.bos_token, inputs)
+
+    assert result == ["hello world", "second string"]
+
+
+def test_a_real_bos_is_still_prepended_to_a_string(bos_tokenizer) -> None:
+    """The guard must not disturb the case the helper exists for."""
+    result = get_input_with_manually_prepended_bos(bos_tokenizer.bos_token, "hello world")
+
+    assert result == bos_tokenizer.bos_token + "hello world"
+
+
+def test_a_real_bos_is_still_prepended_to_a_list(bos_tokenizer) -> None:
+    bos = bos_tokenizer.bos_token
+
+    result = get_input_with_manually_prepended_bos(bos, ["hello world", "second string"])
+
+    assert result == [bos + "hello world", bos + "second string"]
diff --git a/transformer_lens/utilities/tokenize_utils.py b/transformer_lens/utilities/tokenize_utils.py
index 418694798..92f652e6b 100644
--- a/transformer_lens/utilities/tokenize_utils.py
+++ b/transformer_lens/utilities/tokenize_utils.py
@@ -183,18 +183,27 @@ def get_tokenizer_with_bos(tokenizer: PreTrainedTokenizerBase) -> PreTrainedToke
 
 
 def get_input_with_manually_prepended_bos(
-    bos_token: str, input: str | list[str]
+    bos_token: Optional[str], input: str | list[str]
 ) -> str | list[str]:
     """
     Manually prepends the bos token to the input.
 
     Args:
-        bos_token (str): The BOS token to prepend.
+        bos_token (Optional[str]): The BOS token to prepend, or None for a tokenizer
+            that has none (e.g. BERT, T5).
         input (str | list[str]): The input to prepend the bos token to.
 
     Returns:
-        str | list[str]: The input with the bos token manually prepended.
+        str | list[str]: The input with the bos token manually prepended, or unchanged
+            when there is no BOS token to prepend.
     """
+    if bos_token is None:
+        # Nothing to prepend. Callers reach this when prepend_bos is asked for and
+        # cfg.tokenizer_prepends_bos is False — correctly so for a BOS-less tokenizer,
+        # since detect_tokenizer_bos_eos() requires a bos_token_id. Concatenating
+        # would raise a TypeError naming neither the tokenizer nor the flag.
+        return input
+
     if isinstance(input, str):
         input = bos_token + input
     else: