diff --git a/backtestingfx/backtest.py b/backtestingfx/backtest.py
index 4994666..7ff87c6 100644
--- a/backtestingfx/backtest.py
+++ b/backtestingfx/backtest.py
@@ -91,6 +91,11 @@ class Backtest:
         self._quote_to_account = quote_to_account
 
     def _to_bars(self):
+        required = {"open", "high", "low", "close"}
+        missing = required - set(self._df.columns.str.lower())
+        if missing:
+            raise ValueError(f"DataFrame missing required columns: {sorted(missing)}")
+
         bars = []
         for idx, row in self._df.iterrows():
             if isinstance(idx, pd.Timestamp):
diff --git a/src/types.rs b/src/types.rs
index 600ba9b..5597377 100644
--- a/src/types.rs
+++ b/src/types.rs
@@ -53,6 +53,19 @@ pub struct Position {
     pub take_profit: Option<f64>,
 }
 
+#[pymethods]
+impl Position {
+    fn __repr__(&self) -> String {
+        format!(
+            "Position(id={}, {} {:.2} lots @ {:.5})",
+            self.id,
+            if self.is_long { "LONG" } else { "SHORT" },
+            self.lot_size,
+            self.entry_price,
+        )
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct Trade {
     // the actual trade