diff --git a/cartographer/mapping_2d/probability_grid.h b/cartographer/mapping_2d/probability_grid.h
index 2d7e4ccd74..e72a34d8b8 100644
--- a/cartographer/mapping_2d/probability_grid.h
+++ b/cartographer/mapping_2d/probability_grid.h
@@ -171,7 +171,7 @@ class ProbabilityGrid {
     }
   }
 
-  proto::ProbabilityGrid ToProto() {
+  proto::ProbabilityGrid ToProto() const {
     proto::ProbabilityGrid result;
     *result.mutable_limits() = cartographer::mapping_2d::ToProto(limits_);
     result.mutable_cells()->Reserve(cells_.size());