diff --git a/app/models/pageflow/linkmap_page/color_map_file.rb b/app/models/pageflow/linkmap_page/color_map_file.rb
index 49d3391..264f353 100644
--- a/app/models/pageflow/linkmap_page/color_map_file.rb
+++ b/app/models/pageflow/linkmap_page/color_map_file.rb
@@ -46,7 +46,7 @@ class ColorMapFile < GeneratedImageFile
 
       do_not_validate_attachment_file_type :attachment
 
-      serialize :attachment_colors
+      serialize :attachment_colors, Object, coder: YAML
 
       def sprite_url
         attachment.url(:sprite)