diff --git a/src/ssvc/decision_tables/helpers.py b/src/ssvc/decision_tables/helpers.py
index 4326236e..64658de0 100644
--- a/src/ssvc/decision_tables/helpers.py
+++ b/src/ssvc/decision_tables/helpers.py
@@ -377,7 +377,7 @@ def ascii_tree(dt: DecisionTable, df: pd.DataFrame | None = None) -> str:
     Reads a Pandas data frame, builds a decision tree, and returns its ASCII representation.
     """
     # Check for the optional 'row' column and drop it if it exists.
-    if df == None:
+    if df is None:
         df = decision_table_to_longform_df(dt)
 
     if "row" in df.columns: