NoMethodError in Users#new

Showing .../app/views/users/form/_new.haml where line #7 raised:

undefined method `on' for #<ActiveModel::Errors:0x007fb599ec6610>

第7行中的代码是：

7:   = errors_for user,:first_name

而application_helper.rb：

def errors_for(model,field)
  error = case errors = model.errors.on(field)
  ...
end

‘on’是一个default method in ActiveRecord.为什么这不工作？