diff --git a/app/services/backtesting/ui_service.py b/app/services/backtesting/ui_service.py
index ea27ab5..9415c9c 100644
--- a/app/services/backtesting/ui_service.py
+++ b/app/services/backtesting/ui_service.py
@@ -135,7 +135,7 @@ class BacktestPageService:
         return self._yfinance_provider
 
     def get_historical_prices(
-        self, symbol: str, start_date: date, end_date: date, data_source: str = "synthetic"
+        self, symbol: str, start_date: date, end_date: date, data_source: str
     ) -> list[DailyClosePoint]:
         """Load historical prices from the specified data source.