REST API Gateway with lambda, custom integration response http status

I'm attempting to return a custom integration response with a 400 status code from a Python Lambda function for the first time. However, I may be overlooking something related to the status code regex or missing an important detail in this process. I'm also trying to understand which data on runtime is being validated by regex. Thank you very much.

 return {
    'statusCode': 400,
    'errorMessage': 'some error'
}

the default response is disabled

added 400 response

it return this, but expected 400