diff --git a/libs/langchain/langchain/chat_models/openai.py b/libs/langchain/langchain/chat_models/openai.py
index de4108e069..7e1932165b 100644
--- a/libs/langchain/langchain/chat_models/openai.py
+++ b/libs/langchain/langchain/chat_models/openai.py
@@ -570,7 +570,9 @@ class ChatOpenAI(BaseChatModel):
         for message in messages_dict:
             num_tokens += tokens_per_message
             for key, value in message.items():
-                num_tokens += len(encoding.encode(value))
+                # Cast str(value) in case the message value is not a string
+                # This occurs with function messages
+                num_tokens += len(encoding.encode(str(value)))
                 if key == "name":
                     num_tokens += tokens_per_name
         # every reply is primed with <im_start>assistant