Assistant API response does not meet the requirements of my prompt when none of the files match the user's question

I ask the Assistant to return JSON structure as follows:

{
  "is_file_exist_for_user_question": boolean,
  "answer": string
}

When the user question match any file, Assistant will return correct json schema. However if no file matches, it just return one phrase: “Sorry I can’t find the any content matched the question”.