diff --git a/cartographer/mapping/trajectory_node.h b/cartographer/mapping/trajectory_node.h
index a5d8a1a..bf58c3d 100644
--- a/cartographer/mapping/trajectory_node.h
+++ b/cartographer/mapping/trajectory_node.h
@@ -29,7 +29,7 @@
 namespace cartographer {
 namespace mapping {
 
-struct Submaps;
+class Submaps;
 
 struct TrajectoryNode {
   struct ConstantData {