diff --git a/seaweed-volume/build.rs b/seaweed-volume/build.rs
index 08d5cb392..a42b8bdfc 100644
--- a/seaweed-volume/build.rs
+++ b/seaweed-volume/build.rs
@@ -3,6 +3,9 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     tonic_build::configure()
         .build_server(true)
         .build_client(true)
+        // filer.proto uses proto3 optional; older protoc (e.g. 3.12 from ubuntu-22.04 apt)
+        // rejects it without this flag, and newer protoc still accepts the flag
+        .protoc_arg("--experimental_allow_proto3_optional")
         .file_descriptor_set_path(out_dir.join("seaweed_descriptor.bin"))
         .compile_protos(
             &[