diff --git a/src/mesh.cpp b/src/mesh.cpp
index 5ab7ac3988b..85fe9046c59 100644
--- a/src/mesh.cpp
+++ b/src/mesh.cpp
@@ -1472,7 +1472,11 @@ RegularMesh::RegularMesh(hid_t group) : StructuredMesh {group}
 
 int RegularMesh::get_index_in_direction(double r, int i) const
 {
-  return std::ceil((r - lower_left_[i]) / width_[i]);
+  if (r == lower_left_[i]) {
+    return 1;
+  } else {
+    return std::ceil((r - lower_left_[i]) / width_[i]);
+  }
 }
 
 const std::string RegularMesh::mesh_type = "regular";
diff --git a/tests/cpp_unit_tests/test_mesh.cpp b/tests/cpp_unit_tests/test_mesh.cpp
index 24c4f77373f..d2090db1f32 100644
--- a/tests/cpp_unit_tests/test_mesh.cpp
+++ b/tests/cpp_unit_tests/test_mesh.cpp
@@ -255,3 +255,57 @@ TEST_CASE("Test multiple meshes HDF5 roundtrip - spherical")
   REQUIRE(regular_mesh_hdf5->lower_left() == regular_mesh_xml->lower_left());
   REQUIRE(regular_mesh_hdf5->upper_right() == regular_mesh_xml->upper_right());
 }
+
+TEST_CASE("Test get_index_in_direction - regular mesh")
+{
+  // The XML data as a string
+  std::string xml_string = R"(
+        <mesh id="1">
+            <dimension>3 4 5</dimension>
+            <lower_left>-2 -3 -5</lower_left>
+            <upper_right>2 3 5</upper_right>
+       </mesh>
+    )";
+
+  // Create a pugixml document object
+  pugi::xml_document doc;
+
+  // Load the XML from the string
+  pugi::xml_parse_result result = doc.load_string(xml_string.c_str());
+
+  pugi::xml_node root = doc.child("mesh");
+
+  auto mesh = RegularMesh(root);
+
+  REQUIRE(mesh.get_index_in_direction(-2.0, 0) == 1);
+  REQUIRE(mesh.get_index_in_direction(-1.99, 0) == 1);
+  REQUIRE(mesh.get_index_in_direction(1.99, 0) == 3);
+  REQUIRE(mesh.get_index_in_direction(2.0, 0) == 3);
+}
+
+TEST_CASE("Test get_index_in_direction - rectilinear mesh")
+{
+  // The XML data as a string
+  std::string xml_string = R"(
+        <mesh id="1" type="rectilinear">
+            <x_grid>0.0 1.0 5.0 10.0</x_grid>
+            <y_grid>-10.0 -5.0 0.0</y_grid>
+            <z_grid>-100.0 0.0 100.0</z_grid>
+        </mesh>
+    )";
+
+  // Create a pugixml document object
+  pugi::xml_document doc;
+
+  // Load the XML from the string
+  pugi::xml_parse_result result = doc.load_string(xml_string.c_str());
+
+  pugi::xml_node root = doc.child("mesh");
+
+  auto mesh = RectilinearMesh(root);
+
+  REQUIRE(mesh.get_index_in_direction(0.0, 0) == 1);
+  REQUIRE(mesh.get_index_in_direction(0.01, 0) == 1);
+  REQUIRE(mesh.get_index_in_direction(9.99, 0) == 3);
+  REQUIRE(mesh.get_index_in_direction(10.0, 0) == 3);
+}
diff --git a/tests/regression_tests/weightwindows/survival_biasing/local/results_true.dat b/tests/regression_tests/weightwindows/survival_biasing/local/results_true.dat
index b457a245c7c..8a4d3368cc4 100644
--- a/tests/regression_tests/weightwindows/survival_biasing/local/results_true.dat
+++ b/tests/regression_tests/weightwindows/survival_biasing/local/results_true.dat
@@ -1 +1 @@
-386e507008ed3c72c6e1a101aafc01cfaaff2c0b10555558d1eab6332441f7b17264b55002d721151bac2f3e7c1a8aac4c5ed243f5270533d171850f985206ed
\ No newline at end of file
+0d7b17d4e364bda2be21feb2e5b2aae4be69fc8ed634c4f45062e8efc61b7bd24dcf448837692fd603afe3756501fe8821d134f2d6289179618f85178ddb8793
\ No newline at end of file
diff --git a/tests/regression_tests/weightwindows/survival_biasing/shared/results_true.dat b/tests/regression_tests/weightwindows/survival_biasing/shared/results_true.dat
index 11e5ffa77f5..2e88e2f0e81 100644
--- a/tests/regression_tests/weightwindows/survival_biasing/shared/results_true.dat
+++ b/tests/regression_tests/weightwindows/survival_biasing/shared/results_true.dat
@@ -1 +1 @@
-d17a437262d3316985fba4b48e21a7fcd5f32ac2d96ef0e66849f567deaeadc1e0f18d5d0bf5e9cab4246dbf823e7f43f249a1f67e928b27ae8c70b89f3cefbf
\ No newline at end of file
+2f1c8efbf6ab0b0c7319fadd7ee8f27b545c1ebab94ebe15cf1133b54227487542966b6d798970c8b37bc19e7041e687687d5285803241bb83f4c8dc40a4d276
\ No newline at end of file