diff --git a/unfold/unfold.py b/unfold/unfold.py
index 1a39f6d..6c7a7e2 100644
--- a/unfold/unfold.py
+++ b/unfold/unfold.py
@@ -1202,7 +1202,9 @@ def format_superstructure_dataframe(self) -> None:
             (self.scenario_df["flow type"] == "biosphere"), "from database"
         ] = self.scenario_df.loc[
             (self.scenario_df["flow type"] == "biosphere"), "from key"
-        ].apply(lambda x: x[0] if x is not None else None)
+        ].apply(
+            lambda x: x[0] if x is not None else None
+        )
 
         self.scenario_df = self.scenario_df[
             [