diff --git a/src/services/missionInventoryUpdateService.ts b/src/services/missionInventoryUpdateService.ts
index ec3ee941..c4a62f2c 100644
--- a/src/services/missionInventoryUpdateService.ts
+++ b/src/services/missionInventoryUpdateService.ts
@@ -674,7 +674,7 @@ export const addMissionInventoryUpdates = async (
                 const calendarProgress = getCalendarProgress(inventory);
                 const currentSeason = getWorldState().KnownCalendarSeasons[0];
                 calendarProgress.SeasonProgress.LastCompletedChallengeDayIdx = currentSeason.Days.findIndex(
-                    x => x.events[0].challenge == value[value.length - 1].challenge
+                    day => day.events.length != 0 && day.events[0].challenge == value[value.length - 1].challenge
                 );
                 checkCalendarChallengeCompletion(calendarProgress, currentSeason);
                 break;