check length before access each item.
string stringFinal = string.Empty;
string[] delimitator = new string[] { "set" };
string[] strings = toBeDivided.Split(delimitator, StringSplitOptions.None);
if (strings.Length > 1)
{
stringFinal = string.Format("{0} set {1}{2}", strings[0], AddedString, strings[1]);
}
return stringFinal;