Error publishing to rosserial node

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I am running a rosserial node on an Arduino, I am using ROS Noetic. I initialize the node using the following:

<launch>
  <node pkg="rosserial_python" type="serial_node.py" name="motor_control" output="screen">
  <param name="~port" value="/dev/ttyACM0" />
  <param name="~baud" value="57600" />
  </node>
</launch>

The connection is established as shown below:

process[motor_control-1]: started with pid [5063]
[INFO] [1632597275.309702]: ROS Serial Python Node
[INFO] [1632597275.358828]: Connecting to /dev/ttyACM0 at 57600 baud
[INFO] [1632597277.494118]: Requesting topics...
[INFO] [1632597277.839086]: Note: subscribe buffer size is 280 bytes
[INFO] [1632597277.847758]: Setup subscriber on velocity_command [geometry_msgs/Twist]

I publish the msg from the command line:

rostopic pub /velocity_command geometry_msgs/Twist  "{linear:  {x: 0.1, y: 0.0, z: 0.0}, angular: {x: 0.0,y: 0.0, z: 0.0}}"

but when I publish a command to the topic that the node subscribes to I get the following error:

Exception in thread Thread-6:
Traceback (most recent call last):
  File "/usr/lib/python3.8/threading.py", line 932, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.8/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "/home/ubuntu/catkin_ws/src/rosserial/rosserial_python/src/rosserial_python/SerialClient.py", line 778, in processWriteQueue
    self._send(topic, msg)
  File "/home/ubuntu/catkin_ws/src/rosserial/rosserial_python/src/rosserial_python/SerialClient.py", line 758, in _send
    topic_bytes = struct.pack('<h', topic)
struct.error: short format requires (-0x7fff - 1) <= number <= 0x7fff
[INFO] [1632597808.464485]: Sending tx stop request

This error seems a bit cryptic and I am not quite sure how to tackle it. Any ideas what it may be due to?

Subscriber:

// Differential drive kinematics conversion to motor control inputs.

#include <ros.h>
#include <geometry_msgs/Twist.h>
#include <std_msgs/Float64.h>
#include <knet_bot_msgs/EncoderSpeed.h>

#include "CytronMotorDriver.h"

// Constant definitions
#define PI 3.1415926535897932384626433832795

// Robot parameters
float gear_ratio = 98.78;
float wheel_radius = 0.08;
float base_length = 0.171;
float power_supply_voltage = 24.0;

// Motor parameters
float max_motor_rpm = 10000;
float max_motor_voltage = 12.0;
float max_wheel_rpm = max_motor_rpm / gear_ratio;
float max_serial_scale_speed = 250.0 / (power_supply_voltage / max_motor_voltage); 
float max_wheel_linear_speed = max_wheel_rpm * 2.0 * PI * wheel_radius / 60.0; // m/sec

// Feedback parameters
float right_motor_rpm;
float left_motor_rpm;

// Configure the motor driver.
CytronMD motor_right(PWM_DIR, 9, 8);  // PWM 1 = Pin 3, DIR 1 = Pin 4.
CytronMD motor_left(PWM_DIR, 11, 13); // PWM 2 = Pin 9, DIR 2 = Pin

// Initialize ROS parameters
ros::NodeHandle nh;

void motor_callback(const geometry_msgs::Twist& velocity_msg){
  nh.loginfo("Message to motor callback received.");

  // Velocity inputs are v_x and w_z. Others are 0.
  float diff_drive_right_speed = 1/wheel_radius * velocity_msg.linear.x + base_length/(2*wheel_radius) * velocity_msg.angular.z;
  float diff_drive_left_speed = 1/wheel_radius * velocity_msg.linear.x - base_length/(2*wheel_radius) * velocity_msg.angular.z;

  // Check speed saturation limits.
  if(abs(diff_drive_right_speed) > max_wheel_linear_speed){
      diff_drive_right_speed = diff_drive_right_speed/abs(diff_drive_right_speed) * max_wheel_linear_speed;
    }
  if(abs(diff_drive_left_speed) > max_wheel_linear_speed){
      diff_drive_left_speed = diff_drive_left_speed/abs(diff_drive_left_speed) * max_wheel_linear_speed;
    }

  // Convert wheel linear speeds to motor angular velocity in RPM
  float motor_right_speed_input_rpm = 60.0 * diff_drive_right_speed * gear_ratio / (2.0 * PI * wheel_radius);
  float motor_left_speed_input_rpm = 60.0 * diff_drive_left_speed * gear_ratio / (2.0 * PI * wheel_radius ...