Voximplant: Not receiving message from "sendMessage" method

I am trying to use Call.sendMessage method to send info from VoxEngine to WebSDK during the call.

VoxEngine scenario:

call.sendMessage("Test message"); 

Browser js:

voxAPI.addEventListener(VoxImplant.Events.MessageReceived, onMessageReceived);
function onMessageReceived(e){
    console.log("onMessageReceived");
    console.log(e.text);
}

VoxEngine log shows that everything is ok (Executing JS command: SendMessage), but browsers don't receive it . Firefox shows warning "Invalid URI. Load of media resource failed.". Chrome shows nothing. What's wrong?