diff --git a/script/gallery.ts b/script/gallery.ts
index 1d0cd39..b6d043b 100644
--- a/script/gallery.ts
+++ b/script/gallery.ts
@@ -9,7 +9,7 @@ let itemsByYear: Map<string, Set<HTMLElement>>;
 let showSingles = false;
 
 
-function fillSets() {
+function fillSets(): [Set<string>, Set<string>] {
   function checkedValues(boxes: Boxes) {
     return new Set(Array.from(boxes).filter(b => b.checked).map(b => b.value));
   }