diff --git a/app/models/katello/alternate_content_source.rb b/app/models/katello/alternate_content_source.rb
index e9e7ac64a4f..57fbdbdca9f 100644
--- a/app/models/katello/alternate_content_source.rb
+++ b/app/models/katello/alternate_content_source.rb
@@ -54,7 +54,7 @@ class AlternateContentSource < Katello::Model
     validates :content_type, inclusion: {
       in: ->(_) { RepositoryTypeManager.defined_repository_types.keys & CONTENT_TYPES },
       allow_blank: false,
-      message: ->(_, _) { _("is not allowed for ACS. Must be one of the following: %s") % (RepositoryTypeManager.defined_repository_types.keys & CONTENT_TYPES).join(',') }
+      message: ->(_, _) { _("is not allowed for ACS. Must be one of the following: %s") % (RepositoryTypeManager.defined_repository_types.keys & CONTENT_TYPES).sort.join(',') }
     }
     validates :content_type, if: -> { rhui? }, inclusion: {
       in: [::Katello::Repository::YUM_TYPE],