cannot replace first symbol of string says im out of index range

 static string Word(string prevLine, string prevWord, string word)
{
    Console.WriteLine("\"{0}\",\"{1}\",\"{2}\",\"{3}\"", word, word[0],prevWord,prevLine);
    Console.WriteLine(!string.IsNullOrEmpty(word));
    Console.WriteLine(string.IsNullOrEmpty(prevLine));
    Console.WriteLine(!prevWord.EndsWith("."));
    Console.WriteLine(word[0]);

    if (!string.IsNullOrEmpty(word) && 
        (string.IsNullOrEmpty(prevLine) || !prevWord.EndsWith(".")))
    {
        word = word.Remove(0);
        word = word.Insert(0, char.ToString(word[0]));
    }
    return word;
}