diff --git a/tests/integration/test_kv_cache.py b/tests/integration/test_kv_cache.py
index a98ba7de6..baab6696a 100644
--- a/tests/integration/test_kv_cache.py
+++ b/tests/integration/test_kv_cache.py
@@ -213,6 +213,28 @@ def test_freeze_cache(pretrained):
     assert not t.allclose(with_cache_logits_1, with_cache_2_logits_1, atol=atol)
 
 
+def test_kv_cache_with_custom_attention_mask(pretrained):
+    model, atol = pretrained
+    prompt_pre = "An apple"
+    prompt_post = " a day keeps junk the"
+    prompt_whole = "An apple a day keeps the"
+    tokens_pre = model.to_tokens(prompt_pre)
+    tokens_post = model.to_tokens(prompt_post, prepend_bos=False)
+    tokens_whole = model.to_tokens(prompt_whole)
+    correct_logits = model(tokens_whole)
+
+    past_kv_cache = HookedTransformerKeyValueCache.init_cache(
+        model.cfg, model.cfg.device, tokens_pre.shape[0]
+    )
+    model(tokens_pre, past_kv_cache=past_kv_cache)
+    exp_logits = model(
+        tokens_post,
+        attention_mask=t.tensor([[1, 1, 1, 0, 1]], device=model.cfg.device),
+        past_kv_cache=past_kv_cache,
+    )
+    assert t.allclose(correct_logits[:, -1], exp_logits[:, -1], atol=atol)
+
+
 def test_kv_cache_and_start_at_layer(pretrained):
     model, atol = pretrained
     pre_prompt = "I went to Staten Island,"
diff --git a/transformer_lens/HookedTransformer.py b/transformer_lens/HookedTransformer.py
index 56096484c..c084c02b1 100644
--- a/transformer_lens/HookedTransformer.py
+++ b/transformer_lens/HookedTransformer.py
@@ -8,6 +8,7 @@
 alteration of activations in individual components like attention heads and MLP layers, facilitating
 a deeper understanding of the internal workings of transformers like GPT-2.
 """
+
 import logging
 import os
 from typing import (
@@ -297,23 +298,25 @@ def input_to_embed(
         if tokens.device.type != self.cfg.device:
             tokens = tokens.to(devices.get_device_for_block_index(0, self.cfg))
 
-        if attention_mask is not None:
+        if (
+            (self.tokenizer and self.tokenizer.padding_side == "left")
+            or attention_mask is not None
+            or past_kv_cache is not None
+        ):
+            # This means we need to have an explicit attention mask.
+            if attention_mask is None:
+                # If the padding side is left or we are using caching, we need to compute the attention
+                # mask for the adjustment of absolute positional embeddings and attention masking so
+                # that pad tokens are not attended.
+                if prepend_bos is USE_DEFAULT_VALUE:
+                    prepend_bos = self.cfg.default_prepend_bos
+                attention_mask = utils.get_attention_mask(self.tokenizer, tokens, prepend_bos)
+
             assert attention_mask.shape == tokens.shape, (
                 f"Attention mask shape {attention_mask.shape} does not match tokens shape "
                 f"{tokens.shape}"
             )
             attention_mask = attention_mask.to(devices.get_device_for_block_index(0, self.cfg))
-        elif (
-            self.tokenizer and self.tokenizer.padding_side == "left"
-        ) or past_kv_cache is not None:
-            # If the padding side is left or we are using caching, we need to compute the attention
-            # mask for the adjustment of absolute positional embeddings and attention masking so
-            # that pad tokens are not attended.
-
-            if prepend_bos is USE_DEFAULT_VALUE:
-                prepend_bos = self.cfg.default_prepend_bos
-            attention_mask = utils.get_attention_mask(self.tokenizer, tokens, prepend_bos)
-
             if past_kv_cache is not None:
                 # past_kv_cache is not None, so we're doing caching.
                 # We need to extend the previous attention_mask.