diff --git a/frontend/src/hooks/useAdmin.ts b/frontend/src/hooks/useAdmin.ts
index a318dfe..10ae371 100644
--- a/frontend/src/hooks/useAdmin.ts
+++ b/frontend/src/hooks/useAdmin.ts
@@ -360,7 +360,8 @@ export function useAcknowledgeSystemEvents() {
   const { addToast } = useToast();
 
   return useMutation({
-    mutationFn: (days = 7) => adminApi.acknowledgeSystemEvents(days),
+    // Explicit number (not optional) so React Query doesn't infer void variables.
+    mutationFn: (days: number) => adminApi.acknowledgeSystemEvents(days),
     onSuccess: (data) => {
       qc.invalidateQueries({ queryKey: ['admin', 'system-events'] });
       qc.invalidateQueries({ queryKey: ['admin', 'system-events-summary'] });