Type error in group_in_sublists template tag

[ljean]

File "coop_cms/templatetags/coop_utils.py", line 282, in group_in_sublists
return [list_of_objs[i:i+subslist_size] for i in range(0, len(list_of_objs), sublist_size)]
TypeError: object of type 'NoneType' has no len()