Weird character before actual json

[mariusrak]

I receive this from server

a["\u0002\u0000{\"__type\":\"signal\",\"m...

(this is visible in chrome's console)
because of that unicode characters \u0002\u0000, the json is invalid.
But how do they get there?
My code looks like this:

const protocol = JsonProtocol.getInstance()
// ...
protocol.send('signal', { data }, connectionId)
// ...

Thanks