diff --git a/include/openmc/dagmc.h b/include/openmc/dagmc.h
index 0a74928b8..5bf264b43 100644
--- a/include/openmc/dagmc.h
+++ b/include/openmc/dagmc.h
@@ -90,7 +90,7 @@ public:
 
   //! Alternative DAGMC universe constructor for external DAGMC instance
   explicit DAGUniverse(std::shared_ptr<moab::DagMC> external_dagmc_ptr,
-    const std::string& filename, bool auto_geom_ids = false,
+    const std::string& filename = "", bool auto_geom_ids = false,
     bool auto_mat_ids = false);
 
   //! Initialize the DAGMC accel. data structures, indices, material
diff --git a/tests/regression_tests/dagmc/external/main.cpp b/tests/regression_tests/dagmc/external/main.cpp
index 6d9c97f59..9f5c1ceeb 100644
--- a/tests/regression_tests/dagmc/external/main.cpp
+++ b/tests/regression_tests/dagmc/external/main.cpp
@@ -50,7 +50,7 @@ int main(int argc, char* argv[])
 
   // Create new DAGMC universe
   openmc::model::universes.push_back(
-    std::make_unique<openmc::DAGUniverse>(dag_ptr, ""));
+    std::make_unique<openmc::DAGUniverse>(dag_ptr));
   model::universe_map[model::universes.back()->id_] =
     model::universes.size() - 1;