diff --git a/tests/test_ohlcv_date_column.py b/tests/test_ohlcv_date_column.py
index 8c1cd76df..c3a243826 100644
--- a/tests/test_ohlcv_date_column.py
+++ b/tests/test_ohlcv_date_column.py
@@ -6,6 +6,8 @@ instead of `Date`, which would otherwise silently drop every indicator.
 
 from __future__ import annotations
 
+import warnings
+
 import pandas as pd
 import pytest
 
@@ -68,3 +70,12 @@ class TestCleanDataframeAcrossVersions:
         df["close_5_sma"]  # triggers calculation
         assert "close_5_sma" in df.columns
         assert df["close_5_sma"].notna().any()
+
+
+@pytest.mark.unit
+def test_cleaning_a_frame_with_undated_rows_writes_to_its_own_copy():
+    raw = pd.DataFrame({"Date": ["2026-01-08", None, "2026-01-09"], "Close": ["1", "2", "x"]})
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        cleaned = ohlcv._clean_dataframe(raw)
+    assert cleaned["Close"].tolist()[0] == 1.0
diff --git a/tradingagents/dataflows/vendors/yahoo/ohlcv.py b/tradingagents/dataflows/vendors/yahoo/ohlcv.py
index 4c4cacbde..6325d6172 100644
--- a/tradingagents/dataflows/vendors/yahoo/ohlcv.py
+++ b/tradingagents/dataflows/vendors/yahoo/ohlcv.py
@@ -105,7 +105,7 @@ def _clean_dataframe(data: pd.DataFrame) -> pd.DataFrame:
     the latest in-range bar (#1201)."""
     data = _ensure_date_column(data)
     data["Date"] = _normalize_dates(data["Date"])
-    data = data.dropna(subset=["Date"])
+    data = data.dropna(subset=["Date"]).copy()
 
     price_cols = [c for c in ["Open", "High", "Low", "Close", "Volume"] if c in data.columns]
     data[price_cols] = data[price_cols].apply(pd.to_numeric, errors="coerce")