Arrays rebels to my power!

I'm trying to understand why this happens!

I declare an istance array of strings, "ds"

 var ds = new String[5];

It was 4 some time ago, and it worked, but now I had to change it to add a variable, I changed the for cycle, to do one iteration more, but it doesn't work anymore!

 for (i = 20 ; i < 25 ; i++)
         {
             ds[i-20] = lines[i];
         }

It gives me "IndexOutOfRangeException: Array index is out of range. (wrapper stelemref) object:stelemref (object,intptr,object)", but that's impossible, lines[i] works because I tryied it with Debug.Log, while ds[4] no, even if the array has 5 elements... (I tryed to write 3000000 but it doesn't work anyway)

It's a bug of Unity or it's just me?