diff --git a/docs/user-guide/tutorials/CTD_transects.ipynb b/docs/user-guide/tutorials/CTD_transects.ipynb
index b51e17ee..46427b65 100644
--- a/docs/user-guide/tutorials/CTD_transects.ipynb
+++ b/docs/user-guide/tutorials/CTD_transects.ipynb
@@ -329,7 +329,9 @@
     "ctd_down = ctd_df.filter(pl.col(\"raising\") == 0)\n",
     "\n",
     "# build 2D arrays: (n_casts x uniform-depth)\n",
-    "z_2d, distances, var_2d = ctd_build_2d_arrays(ctd_down, plot_variable)\n",
+    "z_2d, distances, var_2d = ctd_build_2d_arrays(\n",
+    "    ctd_down, VARIABLES[plot_variable][\"ds_name\"]\n",
+    ")\n",
     "\n",
     "# 1D depth array from the deepest cast\n",
     "deepest_idx = np.argmax(np.sum(~np.isnan(z_2d), axis=1))\n",