diff --git a/src/index.ts b/src/index.ts
index 25e6ea0..4ab3cc6 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -19,7 +19,7 @@ interface RankingInfo {
 }
 
 interface ValueGetterKey<ItemType> {
-  (item: ItemType): string
+  (item: ItemType): string | Array<string>
 }
 interface IndexedItem<ItemType> {
   item: ItemType