diff --git a/backend/src/kyc-upsert-service.ts b/backend/src/kyc-upsert-service.ts
index aeabd083..a191dc83 100644
--- a/backend/src/kyc-upsert-service.ts
+++ b/backend/src/kyc-upsert-service.ts
@@ -8,6 +8,8 @@ export class ValidationError extends Error {
   constructor(message: string) {
     super(message);
     this.name = 'ValidationError';
+    // Restore prototype chain so `instanceof ValidationError` works after transpilation
+    Object.setPrototypeOf(this, new.target.prototype);
   }
 }