Linux USB Driver: "Unknown symbol in module" with insmod - c

Question

I'm currently learning how to create USB drivers for Linux, and I absolutely love it so far! However, I've come across an issue when trying to insert my .ko file. I keep receiving the following error: "Unknown symbol in module".
Below, you will find my code. What is weird is if I comment out the code in my __init function, the module loads just fine, however after going through my code, and comparing it to other USB driver code, I am unable to locate any issues. Could you please help me out? Thanks so much for your help
struct xb1_controller {
struct usb_device *udev;
struct usb_interface *interface;
unsigned char minor;
char *int_in_buffer;
struct usb_endpoint_descriptor *endpoint;
struct urb *int_in_urb;
};
static struct usb_device_id xb1_table [] = {
{USB_DEVICE(xb1_vendor_id, xb1_product_id) },
{}
};
MODULE_DEVICE_TABLE(usb, xb1_table);
static int xb1_open(struct inode *inode, struct file *file) {
//Sys call
}
static int xb1_release(struct inode *inode, struct file *file) {
//Sys call
}
static ssize_t xb1_read(struct file *file, const char __user *user_buf,
size_t count, loff_t *ppos) {
//Sys call
}
static struct file_operations xb1_fops = {
.owner = THIS_MODULE,
.open = xb1_open,
.release = xb1_release,
.read = xb1_read,
};
static struct usb_class_driver xb1_class = {
.name = "xb1driver",
.fops = &xb1_fops,
.minor_base = MINOR_BASE,
};
static int xb1_probe(struct usb_interface *interface, const struct
usb_device_id *id) {
printk(KERN_INFO "xb1driver probe function called");
}
static void xb1_disconnect(struct usb_interface *interface) {
printk(KERN_INFO "xb1driver disconnect function called");
}
static struct usb_driver xb1_driver = {
.name = "xb1driver",
.id_table = xb1_table,
.probe = xb1_probe,
.disconnect = xb1_disconnect,
};
static int __init xb1_init(void) {
int result;
result = usb_register(&xb1_driver);
if(result) {
printk(KERN_INFO "usb_register has failed..");
return 5;
}
return result;
}
static void __exit xb1_exit(void) {
usb_deregister(&xb1_driver);
}
module_init(xb1_init);
module_exit(xb1_exit);