Observation:[tool] is not a valid tool, try one of [tools]

I am building an LangChain agent with SerpAPI to search internet.
I also wants my agent to handle casual conversation by itself without using a tool.

what happening is
sometimes, the model works well it search on the internet properly.
but sometimes it is giving message like

Observation:[tool] is not a valid tool, try one of [tools].

I am using LLMSingleActionAgent.

How do we handle this kind of situation?