ZeroDivisionError on recalculation

[nonZero]

On regenerate, version 1.4.8

Traceback (most recent call last):
  File "/home/udi/code/PhotoCollage/photocollage/gtkgui.py", line 366, in regenerate_layout
    self.render_from_new_collage(new_collage)
  File "/home/udi/code/PhotoCollage/photocollage/gtkgui.py", line 361, in render_from_new_collage
    self.render_preview()
  File "/home/udi/code/PhotoCollage/photocollage/gtkgui.py", line 316, in render_preview
    collage.page.adjust_cols_heights()
  File "/home/udi/code/PhotoCollage/photocollage/collage.py", line 550, in adjust_cols_heights
    c.adjust_height(target_h)
  File "/home/udi/code/PhotoCollage/photocollage/collage.py", line 340, in adjust_height
    alpha = group.h / sum(c.h for c in group.cells)
            ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ZeroDivisionError: float division by zero