diff --git a/libs/core/langchain_core/messages/utils.py b/libs/core/langchain_core/messages/utils.py
index f7d823bf80a..ccdb0b24e9d 100644
--- a/libs/core/langchain_core/messages/utils.py
+++ b/libs/core/langchain_core/messages/utils.py
@@ -948,8 +948,9 @@ def convert_to_openai_messages(
 
     oai_messages: list = []
 
-    if is_single := isinstance(messages, (BaseMessage, dict)):
+    if is_single := isinstance(messages, (BaseMessage, dict, str)):
         messages = [messages]
+
     messages = convert_to_messages(messages)
 
     for i, message in enumerate(messages):
diff --git a/libs/core/tests/unit_tests/messages/test_utils.py b/libs/core/tests/unit_tests/messages/test_utils.py
index 18b95ce75a4..8777b2674c5 100644
--- a/libs/core/tests/unit_tests/messages/test_utils.py
+++ b/libs/core/tests/unit_tests/messages/test_utils.py
@@ -666,6 +666,12 @@ def create_base64_image(format: str = "jpeg") -> str:
     return f"data:image/{format};base64,{data}"
 
 
+def test_convert_to_openai_messages_string() -> None:
+    message = "Hello"
+    result = convert_to_openai_messages(message)
+    assert result == {"role": "user", "content": "Hello"}
+
+
 def test_convert_to_openai_messages_single_message() -> None:
     message = HumanMessage(content="Hello")
     result = convert_to_openai_messages(message)