String index out of range: -1 Nybörjare Java

import javax.swing.*;
public class antalBokstaver{
public static void main (String[] arg) {

String text= JOptionPane.showInputDialog("Skriv en mening.");
String letter = JOptionPane.showInputDialog("Skriv en bokstav");

int result = antalBokstaver(text, letter);

JOptionPane.showMessageDialog(null, "Du skrev in textraden: " + text + "\n"
+ "Och ville söka efter tecknet: " + letter+ "\n"
+ "Tecknet finns " + result + " gånger i texten.");
}

public static int antalBokstaver(String text, String letter) {

int antal = 0;

for (int i = 0; i < letter.length(); i++){
if (text.substring(i,1).equals(letter)){
anumberLetters = numberLetters + 1;

}
}
return numberLetters;
}
}