diff --git a/src/CSET/operators/filters.py b/src/CSET/operators/filters.py
index da99bfd7f..fdd70ffbf 100644
--- a/src/CSET/operators/filters.py
+++ b/src/CSET/operators/filters.py
@@ -38,11 +38,8 @@ def apply_mask(
 
     Returns
     -------
-    A masked field.
-
-    Return type
-    -----------
-    iris.cube.Cube
+    masked_field: iris.cube.Cube
+        A cube of the masked field.
 
     Notes
     -----