diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2373005fe..85fdc1ed0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,8 @@
 # Changelog Fab-manager
 
+- Fix a bug: event reserved places compute error
+- [TODO DEPLOY] `rails fablab:setup:build_places_cache`
+
 ## v6.0.11 2023 July 21
 
 - Fix a bug: incorrect date range in statistics
diff --git a/app/services/slots/places_cache_service.rb b/app/services/slots/places_cache_service.rb
index 65a19b733..789728700 100644
--- a/app/services/slots/places_cache_service.rb
+++ b/app/services/slots/places_cache_service.rb
@@ -32,7 +32,11 @@ class Slots::PlacesCacheService
 
         reserved_places = (reservations[:reservations].count || 0) + (pending[:reservations].count || 0)
         if slot.availability.available_type == 'event'
-          reserved_places = slot.availability.event.nb_total_places - slot.availability.event.nb_free_places
+          reserved_places = if slot.availability.event.nb_total_places.nil?
+                              0
+                            else
+                              slot.availability.event.nb_total_places - slot.availability.event.nb_free_places
+                            end
         end
         places.push({
                       reservable_type: reservable.class.name,