diff --git a/src/delta_barth/analysis/forecast.py b/src/delta_barth/analysis/forecast.py
index a9b54a5..26509f7 100644
--- a/src/delta_barth/analysis/forecast.py
+++ b/src/delta_barth/analysis/forecast.py
@@ -34,7 +34,7 @@ if TYPE_CHECKING:
 
 def sales_per_customer(
     df: pd.DataFrame,
-    kunde: int,
+    customer_id: int,
     min_num_data_points: int = 100,
 ) -> FcResult:
     """_summary_
@@ -59,7 +59,9 @@ def sales_per_customer(
     # TODO change away from nested DataFrames: just use "f_umsatz_fakt"
     # TODO with strong type checks
     df = df.copy()
-    df_firma = df[(df["firma_refid"] == kunde) & (df["beleg_typ"] == 1) & (df["betrag"] > 0)]
+    df_firma = df[
+        (df["firma_refid"] == customer_id) & (df["beleg_typ"] == 1) & (df["betrag"] > 0)
+    ]
 
     for transaction in df_firma["vorgang_refid"].unique():
         cust_data.order.append(transaction)