diff --git a/app/api/missions/route.ts b/app/api/missions/route.ts
index 06af6e22..5c373a39 100644
--- a/app/api/missions/route.ts
+++ b/app/api/missions/route.ts
@@ -317,7 +317,7 @@ export async function POST(request: Request) {
           participation: body.participation || 'default',
           creatorId: creatorId,
           logo: body.logo || null,
-          leantimeProjectId: body.leantimeProjectId || null,
+          leantimeProjectId: body.leantimeProjectId ? String(body.leantimeProjectId) : null,
           outlineCollectionId: body.documentationCollectionId || null,
           rocketChatChannelId: body.rocketchatChannelId || null,
           giteaRepositoryUrl: body.gitRepoUrl || null,