'pygame.Surface' object is not callable?

# ################### #
#                     #
# Matthias Wheelhouse #
#       Advent        #
#                     #
# ################### #

import pygame, sys
from pygame.locals import *
pygame. init()

# set size and name
size = [420, 250]
screen = pygame.display.set_mode((size))
pygame.display.set_caption("Advent")

# screen refresh
clock = pygame.time.Clock()

## setup of the main program loop and it's variables ##
done = False
black = ( 0, 0, 0)
background = pygame.image.load('bg.bmp')
# --- Main Program Loop --- #
while done == False:
    for event in pygame.event.get(): # user did something
        if event.type == pygame.QUIT: # if user clicked close
            done = True # flag done to exit this loop


    # all game logic should go below this comment

    # all code to draw should go below this comment
    screen.fill(black)
    screen.blit(background(0, 0))
    # update screen
    pygame.display.flip()

    # limit to 20 frames per second
    clock.tick(20)

# Close the window and quit
pygame.quit ()