diff --git a/app/models/activity.rb b/app/models/activity.rb
index 0ab14a82e..189cb43a2 100644
--- a/app/models/activity.rb
+++ b/app/models/activity.rb
@@ -288,7 +288,7 @@ def enroll_reservists!
 
     return if spots == 0
 
-    reservistpool = reservists.order(:created_at).to_a # to_a because in-place `select!`
+    reservistpool = reservists.sort_by { |participant| [participant.created_at] }.to_a # to_a because in-place `select!`
 
     # Filter non-masters if masters-only, non-freshmen if freshman-only.
     if filters?