Here’s my function under test

def publish_sns(error_object, client: boto3.client = None) -> dict:
    """Publish SNS message"""
    client = client or boto3.client('sns')
    topic_arn = os.getenv('SNS_TOPIC_ARN', 'some default')
    subject = f'Appointments encountered a new error {error.statusCode}'
    message = (
        f'correlationId: {error.correlationId}, created: {error.created}, statusCode: {error.statusCode}, logEventId: '
        f'{error.logEventId}, xrayTraceId: {error.xrayTraceId}'
    )
    return client.publish(
        TopicArn=topic_arn, 
        Message=message, 
        Subject=subject,
        # I shouldn't need these
        MessageAttributes={}, 
        MessageStructure='string', 
        PhoneNumber='8888888888'
    )

Here’s the test:

@moto.mock_aws
def test_sns_publish(error):
    client = boto3.client('sns')
    result = aws_helpers.publish_sns(error, client)
    assert 'MessageId' in result

Without the optional parameters I get the following error:
botocore.errorfactory.InvalidParameterException

An error occurred (InvalidParameter) when calling the Publish operation: not enough values to unpack (expected 6, got 1)