diff --git a/src/actions/signup.ts b/src/actions/signup.ts
index a4dc089..ca65e0a 100644
--- a/src/actions/signup.ts
+++ b/src/actions/signup.ts
@@ -66,6 +66,15 @@ export const signup = {
         });
       }
 
+      try {
+        await getJavaUuid(input.teamMember);
+      } catch (_) {
+        throw new ActionError({
+          code: 'NOT_FOUND',
+          message: `Der Minecraft Java Account deines Mitspieler mit dem Username ${input.username} wurde nicht gefunden`
+        });
+      }
+
       // check if user is blocked
       if (uuid) {
         const blockedUser = await db.getBlockedUserByUuid({ uuid: uuid });