a couple of issues, Hash#nested null pointer when the first return in the iternation is nil and AddtoSet#persist doesn't handle nil. This is in 3.1.4. I haven't tested on 4.x master, the code seems to have been re-factored quite a bit there anyway.

here's a reduction

class Foo
include Mongoid::Document
embeds_one :bar, autobuild: true

class Bar
include Mongoid::Document
field :beers, type: Array, default: []
end
end

Foo.create.add_to_set('bar.beers', %w(stone pabst))

here's the exception

NoMethodError: undefined method `[]' for nil:NilClass
from /Users/paul/.rvm/gems/ruby-1.9.3-p392@myapp/gems/mongoid-3.1.4/lib/mongoid/extensions/hash.rb:104:in `block in nested'
from /Users/paul/.rvm/gems/ruby-1.9.3-p392@myapp/gems/mongoid-3.1.4/lib/mongoid/extensions/hash.rb:103:in `each'
from /Users/paul/.rvm/gems/ruby-1.9.3-p392@myapp/gems/mongoid-3.1.4/lib/mongoid/extensions/hash.rb:103:in `nested'
from /Users/paul/.rvm/gems/ruby-1.9.3-p392@myapp/gems/mongoid-3.1.4/lib/mongoid/attributes.rb:91:in `read_attribute'
from /Users/paul/.rvm/gems/ruby-1.9.3-p392@myapp/gems/mongoid-3.1.4/lib/mongoid/persistence/atomic/add_to_set.rb:21:in `block in persist'
from /Users/paul/.rvm/gems/ruby-1.9.3-p392@myapp/gems/mongoid-3.1.4/lib/mongoid/persistence/atomic/operation.rb:91:in `prepare'
from /Users/paul/.rvm/gems/ruby-1.9.3-p392@myapp/gems/mongoid-3.1.4/lib/mongoid/persistence/atomic/add_to_set.rb:20:in `persist'
from /Users/paul/.rvm/gems/ruby-1.9.3-p392@myapp/gems/mongoid-3.1.4/lib/mongoid/persistence/atomic.rb:41:in `add_to_set'