diff --git a/app/models/exercise.rb b/app/models/exercise.rb
index 3812e51e..80725981 100644
--- a/app/models/exercise.rb
+++ b/app/models/exercise.rb
@@ -257,7 +257,9 @@ def set_slug_tags
       name.starts_with? 'context-cnxmod:'
     end.map { |cnxmod| cnxmod.sub 'context-cnxmod:', '' }
 
-    desired_slug_hashes = page_uuids.flat_map { |page_uuid| Content.slugs_by_page_uuid[page_uuid] }
+    desired_slug_hashes = page_uuids.flat_map do |page_uuid|
+      Content.slugs_by_page_uuid[page_uuid] || []
+    end
 
     desired_book_slugs = desired_slug_hashes.map { |slug| "book-slug:#{slug[:book]}" }
     desired_page_slugs = desired_slug_hashes.map do |slug|