List index out of range error while creating threads in for loop

Question

I am not too familiar with C# and I need to implement multithreading by creating 5 threads. However, I keep getting an error:

Run-time exception (line 20): Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index

I am not changing the list at all during program execution, so I'm not sure why I keep getting this error. Here's my code:

using System;
using System.Threading;
using System.Collections.Generic;

namespace Parallelization{
    public class Update{
        //creating a lock object of reference type
        private Object LOCK = new Object(); 
        
        public void StartGroup(){
            Console.WriteLine("Restoring groups");
            List<string> Groups = GetGroupsName();
        
            //parallelizing vm groups
            Thread[] threads = new Thread[Groups.Count];
                
            //create a new thread for each vm group
            for(int i = 0; i < Groups.Count; i++){
                threads[i] = new Thread(() => StartParallel(Groups[i]));
                Console.WriteLine("Starting thread {0}", I);              //ERROR
            }
            for(int i = 0; i < Groups.Count; i++){
                threads[i].Start();
            
            }
        }
        
        //callback for each thread
        private void StartParallel(string GroupName){
            try{
                //Restore group
                lock(LOCK){
                    RestoreGroup(GroupName); 
                }
                
            }
            catch(Exception ex){
                Console.WriteLine("Exception encountered! restoration failed");
            }       
        }   
        
        //function to restore each vm group
        private void RestoreGroup(string GroupName){
            Console.WriteLine(GroupName + " restored!");
        }
            
        //function to obtain a list of group names
        private List<string> GetGroupsName(){
            List<string> groups = new List<string>
                {"group1","group2","group3","group4","group5"};
            return groups;
        }   
    }
    
    public class Groups{
        //calling StartGroup
        public static void Main(string[] args){
            Update update = new Update();
            update.StartGroup();
                
        }
    }
}