diff --git a/tasks/task_13_variance_reduction/4_sphere_iterative_per_batch_ww.ipynb b/tasks/task_13_variance_reduction/4_sphere_iterative_per_batch_ww.ipynb
index 5b0425e1..d7948440 100644
--- a/tasks/task_13_variance_reduction/4_sphere_iterative_per_batch_ww.ipynb
+++ b/tasks/task_13_variance_reduction/4_sphere_iterative_per_batch_ww.ipynb
@@ -183,8 +183,9 @@
     "my_tallies = openmc.Tallies()\n",
     "\n",
     "# This spherical mesh tally is used for generating the weight windows.\n",
-    "mesh = openmc.SphericalMesh()\n",
-    "mesh.r_grid = np.linspace(0, outer_surface.r, 5000)\n",
+    "mesh = openmc.SphericalMesh(\n",
+    "    r_grid = np.linspace(0, outer_surface.r, 5000)\n",
+    ")\n",
     "mesh_filter = openmc.MeshFilter(mesh)\n",
     "flux_tally_for_ww = openmc.Tally(name=\"flux tally\")\n",
     "flux_tally_for_ww.filters = [mesh_filter]\n",