diff --git a/libs/core/langchain_core/messages/base.py b/libs/core/langchain_core/messages/base.py
index 605f879e5b8..b1fb57a5cdd 100644
--- a/libs/core/langchain_core/messages/base.py
+++ b/libs/core/langchain_core/messages/base.py
@@ -139,7 +139,7 @@ def merge_content(
         else:
             # If the last element of the first content is a string
             # Add the second content to the last element
-            if isinstance(merged[-1], str):
+            if merged and isinstance(merged[-1], str):
                 merged[-1] += content
             # If second content is an empty string, treat as a no-op
             elif content == "":
diff --git a/libs/core/tests/unit_tests/test_messages.py b/libs/core/tests/unit_tests/test_messages.py
index 3e0d9e31b29..b62803a3b40 100644
--- a/libs/core/tests/unit_tests/test_messages.py
+++ b/libs/core/tests/unit_tests/test_messages.py
@@ -1,5 +1,5 @@
 import unittest
-from typing import List, Type
+from typing import List, Type, Union
 
 import pytest
 
@@ -18,6 +18,7 @@ from langchain_core.messages import (
     ToolMessage,
     convert_to_messages,
     get_buffer_string,
+    merge_content,
     message_chunk_to_message,
     message_to_dict,
     messages_from_dict,
@@ -950,3 +951,27 @@ def test_tool_message_str() -> None:
     expected = "content='foo' tool_call_id='1' artifact={'bar': {'baz': 123}}"
     actual = str(message)
     assert expected == actual
+
+
+@pytest.mark.parametrize(
+    ["first", "others", "expected"],
+    [
+        ("", [""], ""),
+        ("", [[]], [""]),
+        ([], [""], []),
+        ([], [[]], []),
+        ("foo", [""], "foo"),
+        ("foo", [[]], ["foo"]),
+        (["foo"], [""], ["foo"]),
+        (["foo"], [[]], ["foo"]),
+        ("foo", ["bar"], "foobar"),
+        ("foo", [["bar"]], ["foo", "bar"]),
+        (["foo"], ["bar"], ["foobar"]),
+        (["foo"], [["bar"]], ["foo", "bar"]),
+    ],
+)
+def test_merge_content(
+    first: Union[list, str], others: list, expected: Union[list, str]
+) -> None:
+    actual = merge_content(first, *others)
+    assert actual == expected