src/jdk/nashorn/internal/runtime/ScriptObject.java

*** 689,698 ****
--- 689,699 ----
       */
      public void defineOwnProperty(final int index, final Object value) {
          assert isValidArrayIndex(index) : "invalid array index";
          final long longIndex = ArrayIndex.toLongIndex(index);
          doesNotHaveEnsureDelete(longIndex, getArray().length(), false);
+         setArray(getArray().ensure(longIndex));        
          setArray(getArray().set(index, value, false));
      }
  
      private void checkIntegerKey(final String key) {
          final int index = getArrayIndex(key);