You are incorrect. Look at the error message more closely:

    Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 49
        at java.util.ComparableTimSort.pushRun(Unknown Source)
        at java.util.ComparableTimSort.sort(Unknown Source)
        at java.util.Arrays.sort(Unknown Source)
        at Test.main(Test.java:80)

The error occurs inside TimSort, not at

    arrayToSort[sum] = 1;

It's a bug with TimSort going out of bounds (which obviously shouldn't happen ever), not the Test.