diff --git a/cartographer/mapping/2d/pose_graph/constraint_builder_2d.cc b/cartographer/mapping/2d/pose_graph/constraint_builder_2d.cc
index 3ce1631..91982b0 100644
--- a/cartographer/mapping/2d/pose_graph/constraint_builder_2d.cc
+++ b/cartographer/mapping/2d/pose_graph/constraint_builder_2d.cc
@@ -319,7 +319,7 @@ void ConstraintBuilder2D::RegisterMetrics(metrics::FamilyFactory* factory) {
       counts->Add({{"search_region", "global"}, {"matcher", "found"}});
   auto* queue_length = factory->NewGaugeFamily(
       "/mapping/2d/pose_graph/constraint_builder/queue_length", "Queue length");
-  kQueueLengthMetric = queue_length->Add({{}});
+  kQueueLengthMetric = queue_length->Add({});
   auto boundaries = metrics::Histogram::FixedWidth(0.05, 20);
   auto* scores = factory->NewHistogramFamily(
       "/mapping/2d/pose_graph/constraint_builder/scores",
diff --git a/cartographer/mapping/3d/pose_graph/constraint_builder_3d.cc b/cartographer/mapping/3d/pose_graph/constraint_builder_3d.cc
index 3fda925..c1bb459 100644
--- a/cartographer/mapping/3d/pose_graph/constraint_builder_3d.cc
+++ b/cartographer/mapping/3d/pose_graph/constraint_builder_3d.cc
@@ -352,7 +352,7 @@ void ConstraintBuilder3D::RegisterMetrics(metrics::FamilyFactory* factory) {
       counts->Add({{"search_region", "global"}, {"matcher", "found"}});
   auto* queue_length = factory->NewGaugeFamily(
       "/mapping/3d/pose_graph/constraint_builder/queue_length", "Queue length");
-  kQueueLengthMetric = queue_length->Add({{}});
+  kQueueLengthMetric = queue_length->Add({});
   auto boundaries = metrics::Histogram::FixedWidth(0.05, 20);
   auto* scores = factory->NewHistogramFamily(
       "/mapping/3d/pose_graph/constraint_builder/scores",