diff --git a/narwhals/expr.py b/narwhals/expr.py
index aa5b6f84d..92918cd5c 100644
--- a/narwhals/expr.py
+++ b/narwhals/expr.py
@@ -1277,7 +1277,7 @@ def is_null(self) -> Self:
             │ 5    ┆ 5.0 ┆ false     ┆ false     │
             └──────┴─────┴───────────┴───────────┘
 
-            >>> func(df_pa)
+            >>> func(df_pa)  # nan != null for pyarrow
             pyarrow.Table
             a: int64
             b: double