I'm really not under standing why I get this error. "IndexError: list index out of range checkio, 5 <module>, 18"

            def checkio(array):
               answer = 0
                if len(array) != 0:
                    for i in array[::2]:        
                        answer += array[i]
                    answer *= array[-1]
                    return answer
                else:
                    return answer

            /#These "asserts" using only for self-checking and not necessary for auto-testing
            if __name__ == '__main__':
                print('Example:')
                print(checkio([0, 1, 2, 3, 4, 5]))

                assert checkio([0, 1, 2, 3, 4, 5]) == 30, "(0+2+4)*5=30"
                assert checkio([1, 3, 5]) == 30, "(1+5)*5=30"
                assert checkio([6]) == 36, "(6)*6=36"
                assert checkio([]) == 0, "An empty array = 0"
                print("Coding complete? Click 'Check' to review your tests and earn cool rewards!")

Also could someone help point to where I can find out how to insert the code correctly into posts?

https://gist.github.com/MyceliumMark/04ae69cce1c535d0c69a3ece99361657