diff --git a/app/models/exercise.rb b/app/models/exercise.rb
index fa79eb35..9f3ec6aa 100644
--- a/app/models/exercise.rb
+++ b/app/models/exercise.rb
@@ -562,7 +562,7 @@ class Exercise < ApplicationRecord
   end
 
   def valid_main_file?
-    if files.main_files.count > 1
+    if files.count(&:main_file?) > 1
       errors.add(:files,
         I18n.t('activerecord.errors.models.exercise.at_most_one_main_file'))
     end