diff --git a/tonic-interop/src/server.rs b/tonic-interop/src/server.rs
index 647190d..481dccb 100644
--- a/tonic-interop/src/server.rs
+++ b/tonic-interop/src/server.rs
@@ -148,7 +148,8 @@ impl pb::server::TestService for TestService {
         &self,
         _: Streaming<StreamingOutputCallRequest>,
     ) -> Result<Self::HalfDuplexCallStream> {
-        unimplemented!()
+        // TODO: implement half duplex
+        Err(Status::unimplemented("TODO"))
     }
 
     async fn unimplemented_call(&self, _: Request<Empty>) -> Result<Empty> {