diff --git a/cartographer/mapping/internal/2d/pose_graph/constraint_builder_2d.h b/cartographer/mapping/internal/2d/pose_graph/constraint_builder_2d.h
index 3e350a0..703e1ad 100644
--- a/cartographer/mapping/internal/2d/pose_graph/constraint_builder_2d.h
+++ b/cartographer/mapping/internal/2d/pose_graph/constraint_builder_2d.h
@@ -60,9 +60,8 @@ class ConstraintBuilder2D {
   using Constraint = PoseGraphInterface::Constraint;
   using Result = std::vector<Constraint>;
 
-  ConstraintBuilder2D(
-      const pose_graph::proto::ConstraintBuilderOptions& options,
-      common::ThreadPool* thread_pool);
+  ConstraintBuilder2D(const proto::ConstraintBuilderOptions& options,
+                      common::ThreadPool* thread_pool);
   ~ConstraintBuilder2D();
 
   ConstraintBuilder2D(const ConstraintBuilder2D&) = delete;
diff --git a/cartographer/mapping/internal/2d/pose_graph_2d.cc b/cartographer/mapping/internal/2d/pose_graph_2d.cc
index e9fbfa0..c1dfeeb 100644
--- a/cartographer/mapping/internal/2d/pose_graph_2d.cc
+++ b/cartographer/mapping/internal/2d/pose_graph_2d.cc
@@ -480,7 +480,7 @@ void PoseGraph2D::AddNodeFromProto(const transform::Rigid3d& global_pose,
 
 void PoseGraph2D::SetTrajectoryDataFromProto(
     const proto::TrajectoryData& data) {
-  // Not implemented yet in 2D.
+  LOG(ERROR) << "not implemented";
 }
 
 void PoseGraph2D::AddNodeToSubmap(const NodeId& node_id,
diff --git a/cartographer/mapping/map_builder.cc b/cartographer/mapping/map_builder.cc
index be3bfc5..8f74c0b 100644
--- a/cartographer/mapping/map_builder.cc
+++ b/cartographer/mapping/map_builder.cc
@@ -62,6 +62,8 @@ std::vector<std::string> SelectRangeSensorIds(
 
 MapBuilder::MapBuilder(const proto::MapBuilderOptions& options)
     : options_(options), thread_pool_(options.num_background_threads()) {
+  CHECK(options.use_trajectory_builder_2d() ^
+        options.use_trajectory_builder_3d());
   if (options.use_trajectory_builder_2d()) {
     pose_graph_ = common::make_unique<PoseGraph2D>(
         options_.pose_graph_options(),