diff --git a/paddle/fluid/operators/detail/grpc_server.cc b/paddle/fluid/operators/detail/grpc_server.cc
index 14152bded89d3e33e5f5963676c9d19b331c8bbc..5a87258901c6563fe793d4041f344011a56d9a01 100644
--- a/paddle/fluid/operators/detail/grpc_server.cc
+++ b/paddle/fluid/operators/detail/grpc_server.cc
@@ -169,6 +169,7 @@ class RequestPrefetch final : public RequestBase {
 
     auto scope = request_->GetMutableLocalScope();
     auto invar = scope->FindVar(in_var_name);
+    // out var must be created in local scope!
     framework::Variable* outvar = scope->Var(out_var_name);
 
     request_handler_->Handle(in_var_name, scope, invar, &outvar, out_var_name);