ArgumentOutOfRangeException

Ive been looking at a way to split a long string into segments, and come up with the following function:-

        static string[] Split(string str, int chunkSize)
        {
            int arraySize = new int();

            if (str.Length % chunkSize == 0) {
                arraySize = str.Length / chunkSize;
            } else {
                arraySize = (str.Length / chunkSize)+1;
            }

            string[] n = new string[arraySize]; 

            for (int x = 1; x<=arraySize; x++) {
                n[x] = str.Substring((x * chunkSize), (x * (chunkSize - 1)));
            }
            return n;
        }

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Seems to run through fine, for first serveral runs, however then fails saying:-

An unhandled exception of type 'System.ArgumentOutOfRangeException' occurred in mscorlib.dll

I dont understand why its failing cause it doesnt seem to be out of range :-S

Im new to c# and could do with any assistance in pointing out my error.

Thank you