in src/SignalingClient.ts [314:320]
private validateRecipientClientId(recipientClientId?: string): void {
if (this.config.role === Role.MASTER && !recipientClientId) {
throw new Error('Missing recipient client id. As the MASTER, all messages must be sent with a recipient client id.');
} else if (this.config.role === Role.VIEWER && recipientClientId) {
throw new Error('Unexpected recipient client id. As the VIEWER, messages must not be sent with a recipient client id.');
}
}