oksy so I am creating a script to shuffle close targets

here is my code

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class Beta_targeting : MonoBehaviour
{
    public Transform selectedTarget;
    public List<Transform> targets;

    void Start ()
    {
        targets = new List<Transform>();
    }

    public IList<Transform> FindEnemiesInSphere(float radius)
    { 
        Collider[] cols = Physics.OverlapSphere(transform.position, 10);
        SortedList<float, Transform> transforms = new SortedList<float, Transform>();
        foreach (Collider hit in cols) {
            if (hit && hit.tag == "Enemy"){              
                float Dist = Vector3.Distance(transform.position, hit.transform.position);
                transforms.Add(Dist, hit.transform); 
            }
        }
        foreach(Transform trans in FindEnemiesInSphere(6.0f))
        {
            targets.Add(trans);
        }
        return transforms.Values;
    }

    void Update ()
    {
        if(Input.GetKeyDown(KeyCode.Tab))
            choosingtarget();
    }

    public void choosingtarget()
    {
        if(selectedTarget == null) {
            selectedTarget = targets[0];
        }
        else {
            int index = targets.IndexOf(selectedTarget);
            if(index < targets.Count - 1) {
                index++;
            }
            else {
                index = 0;
            }
            selectedTarget = targets[index];
        }
    }   
}

however I get an error

"Argument is out of range. Parameter name: index"

any help?