# controller for group/customers
def index
  @customers = current_user.available_customers(param[:group_id].to_i)
end

# user
def available_customers(group_id)
  # some other code, that sometimes returns early

  # this is the line of failure
  # self.accessible_group_ids returns an array of integers
  # as does Group.children_ids
  # This line is attempting to get the subset of group ids that the user
  # is *allowed* to touch, and the hierarchy of group ids under the selected one.
  ids = (self.accessible_group_ids & ([group_id] + Group.children_ids(group_id)))
  return Customer.where(group_id: ids)
end

def available_customers(group_id)
  # nil handling
  group_id = group_id.to_i
  # everything else
end