diff --git a/narwhals/series.py b/narwhals/series.py
index 2df55aa9e..19dd35933 100644
--- a/narwhals/series.py
+++ b/narwhals/series.py
@@ -1437,7 +1437,7 @@ def cum_sum(self: Self, *, reverse: bool = False) -> Self:
             reverse: reverse the operation
 
         Returns:
-            A new Series with the cumulative sum of the values.
+            A new Series with the cumulative sum of non-null values.
 
         Examples:
             >>> import pandas as pd