[pygame] Help with pygame.transform.threshold

[Thor Julsrud <thrstein@xxxxxxxxx>]

Hi!
I'm trying to use the threshold function in a roguelike to get appropriate 
background and foreground colors for tiles (simulating ASCII). I've simply 
used pygame.transform.threshold(surface1, surface2, WHITE, (0,0,0), RED, 1, 
None, True). 
However, it keeps complaining "TypeError: must be pygame.Surface, not None". 
If I change the None to a Surface it works, but obviously not the way I 
intended to use the function... Is this simply a bug, or am I doing something 
wrong?
Thanks.