diff --git a/Source/WarpX.cpp b/Source/WarpX.cpp
index c01f26aac63..8b4586aeb05 100644
--- a/Source/WarpX.cpp
+++ b/Source/WarpX.cpp
@@ -89,8 +89,8 @@ Vector<Real> WarpX::E_external_grid(3, 0.0);
 Vector<Real> WarpX::B_external_grid(3, 0.0);
 
 std::string WarpX::authors;
-std::string WarpX::B_ext_grid_s = "default";
-std::string WarpX::E_ext_grid_s = "default";
+std::string WarpX::B_ext_grid_s;
+std::string WarpX::E_ext_grid_s;
 bool WarpX::add_external_E_field = false;
 bool WarpX::add_external_B_field = false;