NoMethodError in Rich::FilesController#index

[JasperH8g]

I'm having trouble to get the Picker to work in RailsAdmin. This is my environment:

• Ruby 2.2.1
• Rails 4.2.0
• RailsAdmin 0.6.7

edit do
  field :string_field, :rich_picker
  field :header_image, :rich_picker
end

Both show a link to select an image, but when I click on it I get this error:

NoMethodError in Rich::FilesController#index
undefined method `attr_accessible' for #<Class:0x007fd2d4038960>

Extracted source (around line #26):
24        send(name, *arguments, &block)
25      else
26        super
27      end
28    end