Pygame animation - IndexError: list index out of range

I'm creating an enemy class for my game using pygame. I have 9 images for my enemy which I animate so that it looks like the enemy is actually moving. I wrote my code down but when I ran it, it said:

IndexError: list index out of range

Can anyone help me to figure out what I have to change in my code? Thanks beforehand. The error is at the line self.image = self.imagesright[self.frame//self.ani].

Here's my Enemy class:

class Enemy(pygame.sprite.Sprite):
    '''
    Spawn an enemy
    '''
    def __init__(self, enemy_list):
        pygame.sprite.Sprite.__init__(self)
        self.health = 50
        self.frame = 0
        self.alpha = (0,0,0)
        self.ani = 2 # animation cycles
        self.enemy_list = enemy_list
        self.add(self.enemy_list)
        self.counter = 0 # counter variable
        self.imagesleft = []
        self.imagesright = []
        for i in range(1,10):
            img = pygame.image.load(os.path.join('images','Bot' + str(i) + '.png')).convert()
            img.convert_alpha()
            img.set_colorkey(self.alpha)
            self.imagesleft.append(img)
            self.image = self.imagesleft[0]
            self.rect  = self.image.get_rect()
        for i in range(1,10):
            img = pygame.image.load(os.path.join('images','Bot' + str(i) + '.png')).convert()
            img = pygame.transform.flip(img, True, False)
            img.convert_alpha()
            img.set_colorkey(self.alpha)
            self.imagesright.append(img)
            self.image = self.imagesright[0]
            self.rect  = self.image.get_rect()

    def move(self):
        '''
        enemy movement
        '''
        distance = 30
        speed = 10

        if self.counter >= 0 and self.counter <= distance:
            self.rect.x += speed
            self.frame += 1
            if self.frame > 9*self.ani:
                self.frame = 0
            self.image = self.imagesright[self.frame//self.ani]

        elif self.counter >= distance and self.counter <= distance*2:
            self.rect.x -= speed
            self.frame += 1
            if self.frame > 9*self.ani:
                self.frame = 0
            self.image = self.imagesleft[self.frame//self.ani]
        else:
            self.counter = 0

        self.counter += 1

    def update(self, dt, all_sprites):
        bullet_list = pygame.sprite.spritecollide(self, all_sprites, True)
        for bullets in bullet_list:
            self.health -= 10
            print(self.health)
        if self.health <= 0:
            self.kill()