diff --git a/backend/backend_api/models.py b/backend/backend_api/models.py
index 7a65713..e61ab37 100644
--- a/backend/backend_api/models.py
+++ b/backend/backend_api/models.py
@@ -139,7 +139,7 @@ def participants(self):
         for participant in WorkshopRegistration.objects.filter(workshop=self,
                                                                status=
                                                                WorkshopRegistration.StatusChoices.PURCHASED):
-            participants += participant.user
+            participants.append(participant.user)
         return participants
 
     @property
@@ -210,7 +210,7 @@ def participants(self):
         for participant in PresentationParticipation.objects.filter(presentation=self,
                                                                     status=
                                                                     PresentationParticipation.StatusChoices.PURCHASED):
-            participants += participant.user
+            participants.append(participant.user)
         return participants
 
     @property