diff --git a/src/CSET/operators/transect.py b/src/CSET/operators/transect.py
index 0dd357653..ab19e05b0 100644
--- a/src/CSET/operators/transect.py
+++ b/src/CSET/operators/transect.py
@@ -165,6 +165,8 @@ def calc_transect(cube: iris.cube.Cube, startcoords: tuple, endcoords: tuple):
     interpolated_cubes[0].attributes["transect_coords"] = (
         f"{startcoords[0]}_{startcoords[1]}_{endcoords[1]}_{endcoords[1]}"
     )
+
+    # optimise above
     # If concatenation successful, should be CubeList with one cube left.
     assert len(interpolated_cubes) == 1
     return interpolated_cubes[0]