[RESOLVED] Uppercase first letter

[vbbit]

Can someone tells me what's wrong with my code:

        protected string CapFirstLetter(string sentence)
        {
            sentence = char.ToUpper(sentence[0]) + sentence.Substring(1);
            return sentence;
        }

Then I call CapFirstLetter(Mytextbox.text)

It never prints the first letter upper case.