Ohm error

Exception

NoMethodError: protected method `key_for' called for Activity:Class

# Stacktrace

lib/ohm_model_template.rb:65:in `method_missing'
[GEM_ROOT]/bundler/gems/ohm-870c88ac4614/lib/ohm.rb:741:in `union_key_for'
[GEM_ROOT]/bundler/gems/ohm-870c88ac4614/lib/ohm.rb:732:in `block (2 levels) in keys'
[GEM_ROOT]/bundler/gems/ohm-870c88ac4614/lib/ohm.rb:723:in `each'
[GEM_ROOT]/bundler/gems/ohm-870c88ac4614/lib/ohm.rb:723:in `block in keys'
[GEM_ROOT]/bundler/gems/ohm-870c88ac4614/lib/ohm.rb:722:in `tap'
[GEM_ROOT]/bundler/gems/ohm-870c88ac4614/lib/ohm.rb:722:in `keys'
[GEM_ROOT]/bundler/gems/ohm-870c88ac4614/lib/ohm.rb:710:in `find_source'
[GEM_ROOT]/bundler/gems/ohm-870c88ac4614/lib/ohm.rb:605:in `find'
[GEM_ROOT]/bundler/gems/ohm-870c88ac4614/lib/ohm.rb:784:in `find'
[GEM_ROOT]/bundler/gems/ohm-870c88ac4614/lib/ohm.rb:1456:in `find'
[...]

# Line that causes the exception

Activity.find(:admin_user_id => [1, 2, 3])

ohm_model_template.rb

module OhmModelTemplate
  extend ActiveSupport::Concern

  [...]

  module ClassMethods
    [...]

    def method_missing(name, *args, &block)
      if [...]
      else
        super name, *args, &block
      end
    end
  end
end

class Activity < Ohm::Model
  include OhmModelTemplate

  [...]
end