diff --git a/src/compiler/protobuf_plugin.h b/src/compiler/protobuf_plugin.h
index e1407ffc422..cb01bd34987 100644
--- a/src/compiler/protobuf_plugin.h
+++ b/src/compiler/protobuf_plugin.h
@@ -73,7 +73,7 @@ class ProtoBufMethod : public grpc_generator::Method {
   bool get_module_and_message_path_input(grpc::string *str,
                                          grpc::string generator_file_name,
                                          bool generate_in_pb2_grpc,
-                                         grpc::string &import_prefix) const {
+                                         grpc::string import_prefix) const {
     return grpc_python_generator::GetModuleAndMessagePath(
         method_->input_type(), str, generator_file_name, generate_in_pb2_grpc,
         import_prefix);
@@ -82,7 +82,7 @@ class ProtoBufMethod : public grpc_generator::Method {
   bool get_module_and_message_path_output(grpc::string *str,
                                           grpc::string generator_file_name,
                                           bool generate_in_pb2_grpc,
-                                          grpc::string &import_prefix) const {
+                                          grpc::string import_prefix) const {
     return grpc_python_generator::GetModuleAndMessagePath(
         method_->output_type(), str, generator_file_name, generate_in_pb2_grpc,
         import_prefix);
diff --git a/src/compiler/schema_interface.h b/src/compiler/schema_interface.h
index 41e9322a0d0..25bbdb51420 100644
--- a/src/compiler/schema_interface.h
+++ b/src/compiler/schema_interface.h
@@ -72,10 +72,10 @@ struct Method : public CommentHolder {
 
   virtual bool get_module_and_message_path_input(
       grpc::string *str, grpc::string generator_file_name,
-      bool generate_in_pb2_grpc) const = 0;
+      bool generate_in_pb2_grpc, grpc::string import_prefix) const = 0;
   virtual bool get_module_and_message_path_output(
       grpc::string *str, grpc::string generator_file_name,
-      bool generate_in_pb2_grpc) const = 0;
+      bool generate_in_pb2_grpc, grpc::string import_prefix) const = 0;
 
   virtual grpc::string get_input_type_name() const = 0;
   virtual grpc::string get_output_type_name() const = 0;