diff --git a/cartographer/mapping/internal/2d/pose_graph_2d.cc b/cartographer/mapping/internal/2d/pose_graph_2d.cc
index 5f12521..79862a6 100644
--- a/cartographer/mapping/internal/2d/pose_graph_2d.cc
+++ b/cartographer/mapping/internal/2d/pose_graph_2d.cc
@@ -392,6 +392,7 @@ void PoseGraph2D::WaitForAllComputations() {
 }
 
 void PoseGraph2D::FinishTrajectory(const int trajectory_id) {
+  common::MutexLocker locker(&mutex_);
   AddWorkItem([this, trajectory_id]() REQUIRES(mutex_) {
     CHECK_EQ(finished_trajectories_.count(trajectory_id), 0);
     finished_trajectories_.insert(trajectory_id);
diff --git a/cartographer/mapping/internal/3d/pose_graph_3d.cc b/cartographer/mapping/internal/3d/pose_graph_3d.cc
index e9cc96d..7c81fd8 100644
--- a/cartographer/mapping/internal/3d/pose_graph_3d.cc
+++ b/cartographer/mapping/internal/3d/pose_graph_3d.cc
@@ -410,6 +410,7 @@ void PoseGraph3D::WaitForAllComputations() {
 }
 
 void PoseGraph3D::FinishTrajectory(const int trajectory_id) {
+  common::MutexLocker locker(&mutex_);
   AddWorkItem([this, trajectory_id]() REQUIRES(mutex_) {
     CHECK_EQ(finished_trajectories_.count(trajectory_id), 0);
     finished_trajectories_.insert(trajectory_id);