diff --git a/frontend/src/app/distribution/_components/DeliveryEditPage.tsx b/frontend/src/app/distribution/_components/DeliveryEditPage.tsx
index 07af0a8..a4637bb 100644
--- a/frontend/src/app/distribution/_components/DeliveryEditPage.tsx
+++ b/frontend/src/app/distribution/_components/DeliveryEditPage.tsx
@@ -239,7 +239,7 @@ export default function DeliveryEditPage({ planId }: Props) {
   // 未割り当て圃場（グループに入っていない + 選択肥料のエントリがある）
   const unassignedFields = useMemo(() => {
     const result: FieldInfo[] = [];
-    for (const fId of relevantFieldIds) {
+    for (const fId of Array.from(relevantFieldIds)) {
       if (!allGroupFieldIds.has(fId)) {
         result.push(getFieldInfo(fId));
       }