irb(main):002:0> Band.where(nil)
Traceback (most recent call last):
        2: from (irb):1
        1: from (irb):2:in `rescue in irb_binding'
Mongoid::Errors::CriteriaArgumentRequired ()
message:
  Calling Criteria methods with nil arguments is not allowed.
summary:
  Arguments to Criteria methods cannot be nil, and most Criteria methods require at least one argument. Only logical operations (and, or, nor and not), `all' and `where' can be called without arguments. The method that was called with nil argument was: where.
resolution:
  Invoke Criteria methods with non-nil arguments or omit the respective invocations.

irb(main):003:0> Band.where     
Traceback (most recent call last):
        2: from (irb):2
        1: from (irb):3:in `rescue in irb_binding'
ArgumentError (wrong number of arguments (given 0, expected 1))