How to remove [Symbol] fields from winston@3 logger output using winston-papertrail?

[dponfl]

On the output I got unexpected like:
[Symbol(level)]: 'warn',
[Symbol(splat)]:
[Symbol(message)]:

How I can avoid this?

I'm using:
new Papertrail({
host: 'url',
port: 123,
level: 'warn',
hostname: '***',
program: 'Server',
logFormat: (level, message) => {
return ${level}: ${message};
},
})