diff --git a/aos/flatbuffers/builder.h b/aos/flatbuffers/builder.h
index 75e0fe2..db89d10 100644
--- a/aos/flatbuffers/builder.h
+++ b/aos/flatbuffers/builder.h
@@ -47,7 +47,8 @@
                     this) {
     SetPrefix();
   }
-  Builder(std::unique_ptr<Allocator> allocator)
+  Builder(std::unique_ptr<Allocator> allocator =
+              std::make_unique<VectorAllocator>())
       : ResizeableObject(
             allocator->AllocateOrDie(kBufferSize, T::kAlign, SetZero::kYes),
             std::move(allocator)),