diff --git a/cartographer_ros/cartographer_ros/node_main.cc b/cartographer_ros/cartographer_ros/node_main.cc
index 0b1707b..10a6612 100644
--- a/cartographer_ros/cartographer_ros/node_main.cc
+++ b/cartographer_ros/cartographer_ros/node_main.cc
@@ -297,7 +297,7 @@ bool Node::HandleSubmapQuery(
 
   carto::common::MutexLocker lock(&mutex_);
   // TODO(hrapp): return error messages and extract common code from MapBuilder.
-  carto::mapping::Submaps* const submaps =
+  const carto::mapping::Submaps* const submaps =
       map_builder_.GetTrajectoryBuilder(kTrajectoryBuilderId)->submaps();
   if (request.submap_id < 0 || request.submap_id >= submaps->size()) {
     return false;