diff --git a/tests/test_components/test_autograd.py b/tests/test_components/test_autograd.py
index 38a2a28a1..fdcfb058f 100644
--- a/tests/test_components/test_autograd.py
+++ b/tests/test_components/test_autograd.py
@@ -37,7 +37,7 @@
 
 # whether to run numerical gradient tests, off by default because it runs real simulations
 RUN_NUMERICAL = False
-_NUMERICAL_COMBINATION = ("center_list", "mode")
+_NUMERICAL_COMBINATION = ("size_element", "mode")
 
 TEST_MODES = ("pipeline", "adjoint", "speed")
 TEST_MODE = "speed" if TEST_POLYSLAB_SPEED else "pipeline"
diff --git a/tidy3d/components/geometry/base.py b/tidy3d/components/geometry/base.py
index 28df913f1..009f0b8ed 100644
--- a/tidy3d/components/geometry/base.py
+++ b/tidy3d/components/geometry/base.py
@@ -2448,7 +2448,7 @@ def derivative_face(
             if min_max_index == 0:
                 index_out, index_in = (0, num_cells_in - 1)
             else:
-                index_out, index_in = (-1, -num_cells_in)
+                index_out, index_in = (-1, -num_cells_in - 1)
             eps_xyz_inside = [eps.isel(**{dim_normal: index_in}) for eps in eps_xyz]
             eps_xyz_outside = [eps.isel(**{dim_normal: index_out}) for eps in eps_xyz]