diff --git a/cartographer_ros/cartographer_ros/occupancy_grid_node_main.cc b/cartographer_ros/cartographer_ros/occupancy_grid_node_main.cc
index 0a3d51f..1703d66 100644
--- a/cartographer_ros/cartographer_ros/occupancy_grid_node_main.cc
+++ b/cartographer_ros/cartographer_ros/occupancy_grid_node_main.cc
@@ -164,11 +164,10 @@ void Node::HandleSubmapList(
 }
 
 void Node::DrawAndPublish(const ::ros::WallTimerEvent& unused_timer_event) {
+  absl::MutexLock locker(&mutex_);
   if (submap_slices_.empty() || last_frame_id_.empty()) {
     return;
   }
-
-  absl::MutexLock locker(&mutex_);
   auto painted_slices = PaintSubmapSlices(submap_slices_, resolution_);
   std::unique_ptr<nav_msgs::OccupancyGrid> msg_ptr = CreateOccupancyGridMsg(
       painted_slices, resolution_, last_frame_id_, last_timestamp_);