pygame.mask.from_surface

One of my students was doing pixel perfect collisions with Sprites and
found what may be a bug.  pygame.mask.from_surface seemed to return an
empty mask when everything on a surface is opaque.  In this situation
rectangle collisions made more sense but I am still curious why
pygame.mask.from_surface is behaving this way.  I quickly dug through
the source and nothing popped out as the cause.