Empty observation when using python_repl

[tomsib2001]

import os
os.environ["OPENAI_API_KEY"] = "..."
from langchain.agents import Tool, load_tools
from langchain.memory import ConversationBufferMemory
from langchain import OpenAI
from langchain.utilities import GoogleSearchAPIWrapper
from langchain.agents import initialize_agent

tool_names = []
tool_names.append("python_repl")

llm=OpenAI(temperature=0.1,model_name="gpt-3.5-turbo")

tools = load_tools(tool_names=tool_names, llm=llm)


memory = ConversationBufferMemory(memory_key="chat_history")
agent_chain = initialize_agent(tools, llm, agent="conversational-react-description", verbose=True, memory=memory)
default_arg = "Hi, how may I help you?"

if len(sys.argv) < 2:
    arg= default_arg
else:
    arg = sys.argv[1]

while(True):
    agent_chain.run(input=arg)
    arg = input("\nHi, how may I help you?\n")

I run

python3 main_21_03.py "compute 5 * 6 using python"

And I get a loop where the agent is running the right command, but its observation is empty:

compute 5 * 6 using Python


> Entering new AgentExecutor chain...
Thought: Do I need to use a tool? Yes
Action: Python REPL
Action Input: 5 * 6
Observation:
Thought:Do I need to use a tool? Yes
Action: Python REPL
Action Input: 5 * 6
Observation:
[etc...]