diff --git a/README.md b/README.md
index 4ca6e3f..05169a9 100644
--- a/README.md
+++ b/README.md
@@ -375,10 +375,16 @@ which uses [Psych.safe_load](https://www.rubydoc.info/stdlib/psych/Psych.safe_lo
 LabTech requires permitting the following classes using
 [config.active_record.yaml_column_permitted_classes](https://guides.rubyonrails.org/configuring.html#config-active-record-yaml-column-permitted-classes):
 
-- `Symbol`
-- `Time`
+- `ActiveSupport::Duration`
 - `ActiveSupport::TimeWithZone`
 - `ActiveSupport::TimeZone`
+- `Symbol`
+- `Time`
+
+*Note: this list is almost certainly incomplete!*
+
+If you discover new ones, please submit an issue or PR.  (There's also a list
+in ./lib/lab_tech/engine.rb that should be considered canonical.)
 
 ## Contributing
 
diff --git a/lib/lab_tech/engine.rb b/lib/lab_tech/engine.rb
index 2bde555..a7ea1e4 100644
--- a/lib/lab_tech/engine.rb
+++ b/lib/lab_tech/engine.rb
@@ -4,18 +4,29 @@ class Engine < ::Rails::Engine
     config.generators.api_only = true
 
     config.after_initialize do
+      # NOTE: this list also exists in the README; please try to keep them in sync.
       required_serializable_classes = [
         ActiveSupport::Duration,
         ActiveSupport::TimeWithZone,
         ActiveSupport::TimeZone,
-        Time,  
+        Symbol,
+        Time,
       ]
-      
+
       missing_classes = required_serializable_classes - Rails.configuration.active_record.yaml_column_permitted_classes
 
       if missing_classes.any?
-        puts "Please add #{missing_classes.join(', ')} to your Rails.configuration.active_record.yaml_column_permitted_classes.".red      
-        puts "LabTech will break your application horribly unless you do.".red      
+        puts <<~EOF.red
+
+          Please add the following classes to your
+          Rails.configuration.active_record.yaml_column_permitted_classes:
+          #{ missing_classes.map { |klass| "  - #{klass}" }.join('\n') }
+
+          Because LabTech uses ActiveRecord's `serialize` to save experimental
+          results to the database, running experiments is likely to break your
+          application horribly without this configuration.
+
+        EOF
       end
     end
   end