Index and length must refer to a location within the string rand number

I'm trying to pass a random number to a character generator that will then return a random string with the length passed to it.

int rndn = rnd.Next(20, 40);
byte[] strarr = Program.StringToByteArray(Program.GetUniqueKey(rndn));

The StringToByteArray is as follows:

    public static byte[] StringToByteArray(String hex)
    {
        int NumberChars = hex.Length;
        byte[] bytes = new byte[NumberChars / 2];
        for (int i = 0; i < NumberChars; i += 2)
            bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);
        return bytes;
    }

If I pass a number manually to it like so:

byte[] strarr = Program.StringToByteArray(Program.GetUniqueKey(25));

It will work correctly and pass back a random string 25 chars long.

What would be causing this?