ServerTCP getMultipleHoldingRegisters and getMultipleInputRegisters don't work with Promises

[ensehic]

I'm having issues implementing async getMultipleInputRegisters and getMultipleHoldingRegisters. See code below.

import { ServerTCP } from 'modbus-serial';

const vector = {
    getMultipleInputRegisters: (addr: number, length: number, unitID: number) => {
        return new Promise((resolve) => {
            resolve([1, 2]);
        });
    },
    getMultipleHoldingRegisters: (addr: number, length: number, unitID: number) => {
        return new Promise((resolve) => {
            resolve([3, 4]);
        });
    }
};

const serverTCP = new ServerTCP(vector, { host: '0.0.0.0', port: 502, unitID: 1 });
serverTCP.on('error', console.log);

When I attempt to read multiple input registers or holding registers (trying to read, say, address 10 and length 2), I receive this error on the client side:

{
    status: 'rejected',
    reason: Error: Modbus exception 4: Slave device failure (device reports internal error)
        at ModbusRTU._onReceive (...)
        at TcpPort.emit (node:events:518:28)
        at TcpPort.emit (node:domain:488:12)
        at Socket.<anonymous> (...)
        at Socket.emit (node:events:518:28)
        at Socket.emit (node:domain:488:12)
        at addChunk (node:internal/streams/readable:559:12)
        at readableAddChunkPushByteMode (node:internal/streams/readable:510:3)
        at Socket.Readable.push (node:internal/streams/readable:390:5)
        at TCP.onStreamRead (node:internal/stream_base_commons:190:23) {
      modbusCode: 4
    }
}

The error message is rather generic, so I haven't been able to glean anything helpful from it. I have verified that both these methods work when used synchronously. Also all other methods (setCoil, getInputRegister etc.) work fine with Promises. I imagine I'm doing something wrong here, but can't for the life of me figure out what it is. Any help would be appreciated. 🙂