diff --git a/app/api/missions/route.ts b/app/api/missions/route.ts
index d37996d..d121c6e 100644
--- a/app/api/missions/route.ts
+++ b/app/api/missions/route.ts
@@ -293,7 +293,7 @@ export async function POST(request: Request) {
     // Add guardians
     if (body.guardians) {
       for (const guardianId of Object.values(body.guardians)) {
-        if (guardianId) {
+        if (guardianId && typeof guardianId === 'string') {
           missionMemberIds.add(guardianId);
         }
       }