SlackHandler, how to print the value of $record['message']

I am using Monolog in Lumen. I can't figure out how to get just the message of error... i think the problem is here:

``` php
        if ($this->formatter) {
            $message = $this->formatter->format($record);
        } else {
            $message = $record['message'];
        }
```

$this->formatter is never empty... I tried pushing a null formatter, but it doesn't work.
Any ideas?
