Endless quicksort recursive function (academic assignment)

Question

I have a few functions here that are taking strings from an array and sorting them alphabetically. SortList is the main function, which calls functions CompareString, LastName, and FirstName. Functions LastName & FirstName simply take a string containing a full name and parse them for their respective parts. CompareString first looks at every char of two last names and compares them, returning true if the pivot value is less than the target. If the last names are the same, then it looks at both first names.

When I run it, I get the error in the attached screenshot.

public bool CompareString(string lName1, string lName2, string fName1, string fName2)
{
    // While there are letters in the last name
    for (int i = 0; i < lName1.Length && i < lName2.Length; i++)
    {
        // If letter of first string is after letter of pivot string
        if (lName1[i] > lName2[i])
            return false;
        else if (lName1[i] < lName2[i])
            return true;
        // If we get here, last names are the same
        else if (i == lName1.Length - 1 || i == lName2.Length - 1)
        {

            // While there are letters in the first name
            for (int j = 0; j < fName1.Length && j < fName2.Length; j++)
            {
                // If letter of first string is less than letter of pivot string
                if (fName1[j] < fName2[j])
                {
                    return false;
                }
            }

        } // End first name compare

    } // End name compare

    // Pivot string is first alphabetically
    return true;

} // End CompareString


// PURPOSE: Fills array with data from file
// PRE:
// POST: Array filled with data from file
public void SortList(int leftInd, int rightInd)
{
    int l = leftInd;    // initialized with 0 
    int r = rightInd;   // initialized with numDivers - 1

   // DiverRec pivot = diverList[leftInd];       // set to first value
    DiverRec temp;

    // Assign last names to left, right, & pivot
    string lLast = LastName(diverList[l].name);
    string rLast = LastName(diverList[r].name);
    string pLast = LastName(diverList[l].name);

    // Assign first names to left, right, & pivot
    string lFirst = FirstName(diverList[l].name);
    string rFirst = FirstName(diverList[r].name);
    string pFirst = FirstName(diverList[l].name);


    // Compare left & right strings with pivot
    while (l <= r)
    {
        while (CompareString(lLast, pLast, lFirst, pFirst))
        {
            l++;
            lLast = LastName(diverList[l].name);
            lFirst = FirstName(diverList[l].name);
        }

        while (!CompareString(rLast, pLast, rFirst, pFirst))
        {
            r--;
            rLast = LastName(diverList[r].name);
            rFirst = FirstName(diverList[r].name);
        }


        if (l <= r)
        {
            temp = diverList[l];
            diverList[l] = diverList[r];
            diverList[r] = temp;

            l++;
            r--;
        }

    }
   
    if (leftInd < r)
        SortList(leftInd, r);
    if (l < rightInd)
        SortList(l, rightInd);

    /*if (l > r)
    {
        temp = diverList[r];
        diverList[r] = pivot;
        pivot = temp;

        l++;
        r--;
    }

    for (int i = 0; i < numDivers - 1; i++)
        Console.WriteLine(diverList[i].name);*/



} // End SortList


public string LastName(string lName)
{
    char delim = ' ';                   // Separates first & last name
    int pos = lName.IndexOf(delim);     // Position of delim

    // Isolate last name
    string last = lName.Substring(pos + 1, lName.Length - (pos + 1));

    // Return last name
    return last;

} // End LastName
public string FirstName(string fName)
{
    char delim = ' ';                   // Separates first & last name
    int pos = fName.IndexOf(delim);     // Position of delim

    // Isolate first name
    string first = fName.Substring(0, pos);

    // Return first name
    return first;

} // End FirstName

I've tried multiple things, making sure I'm incrementing & decrementing in the right places, making sure the strings are read in correctly... I'm almost certain that the issue is somewhere in the recursive function because I only learned about it last week so I'm not really confident in implementing it yet. Thank you in advance to anyone who's willing to lend a hand.