diff --git a/src/delta_barth/analysis/forecast.py b/src/delta_barth/analysis/forecast.py
index ff16175..87396a3 100644
--- a/src/delta_barth/analysis/forecast.py
+++ b/src/delta_barth/analysis/forecast.py
@@ -251,7 +251,7 @@ def _process_sales(
     # TODO: therefore, stepping with fixed value n does not result in timedelta of n episodes
     # Option A: pad data frame with zero values --> could impede forecast algorithm
     # Option B: calculate next index based on timedelta
-    dates = monthly_sum.index
+    dates = cast(pd.DatetimeIndex, monthly_sum.index)
     # print("dates: ", dates)
     # ?? --- new: use monthly basis for time windows
     # baseline: 3 years - 36 months