diff --git a/qlib/data/ops.py b/qlib/data/ops.py
index d9a2ffbb3e3..f68fd894ece 100644
--- a/qlib/data/ops.py
+++ b/qlib/data/ops.py
@@ -1277,6 +1277,7 @@ def _load_internal(self, instrument, start_index, end_index, *args):
         _series = self.feature.load(instrument, start_index, end_index, *args)
         if self.N == 0:
             series = pd.Series(expanding_rsquare(_series.values), index=_series.index)
+            series.loc[np.isclose(_series.expanding(min_periods=1).std(), 0, atol=2e-05)] = np.nan
         else:
             series = pd.Series(rolling_rsquare(_series.values, self.N), index=_series.index)
             series.loc[np.isclose(_series.rolling(self.N, min_periods=1).std(), 0, atol=2e-05)] = np.nan
diff --git a/tests/ops/test_rsquare.py b/tests/ops/test_rsquare.py
new file mode 100644
index 00000000000..35647223f3f
--- /dev/null
+++ b/tests/ops/test_rsquare.py
@@ -0,0 +1,60 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+import unittest
+
+import numpy as np
+import pandas as pd
+
+from qlib.data.ops import Rsquare
+
+
+class _StubFeature:
+    """Minimal Expression stub: returns a fixed series from ``load`` so that
+    Rolling operators can be exercised without a data provider / market data."""
+
+    def __init__(self, series: pd.Series):
+        self._series = series
+
+    def load(self, *args, **kwargs) -> pd.Series:
+        return self._series
+
+
+class TestRsquareDegenerateWindow(unittest.TestCase):
+    """Regression tests for Rsquare on (near-)constant windows.
+
+    A regression of a (near-)constant series is degenerate: the coefficient of
+    determination is 0/0. The Cython kernels compute it as ``num / sqrt(var_x *
+    var_y)`` where ``var_y ~ 0``, so floating-point cancellation yields ``inf``
+    or a spurious finite value instead of ``NaN``. ``Rsquare`` guards against
+    this by masking windows whose std is ~0 to ``NaN`` -- this must hold for the
+    expanding (``N == 0``) path as well as the rolling (``N != 0``) path.
+    """
+
+    # near-constant: only float-noise variation, std ~ 5e-7 (< the 2e-5 guard atol)
+    SERIES = pd.Series([100.0, 100.0, 100.0, 100.000001, 100.0, 100.0])
+
+    def _rsquare(self, n: int) -> pd.Series:
+        return Rsquare(_StubFeature(self.SERIES), n)._load_internal("SH600000", None, None)
+
+    def test_expanding_window_is_guarded(self):
+        # N == 0 -> expanding. Before the fix this leaked inf / garbage R^2.
+        out = self._rsquare(0)
+        self.assertFalse(np.isinf(out.to_numpy()).any(), f"expanding Rsquare leaked inf: {out.tolist()}")
+        finite = out.dropna()
+        self.assertTrue(((finite >= 0.0) & (finite <= 1.0)).all(), f"R^2 outside [0, 1]: {finite.tolist()}")
+
+    def test_rolling_window_is_guarded(self):
+        out = self._rsquare(4)
+        self.assertFalse(np.isinf(out.to_numpy()).any(), f"rolling Rsquare leaked inf: {out.tolist()}")
+
+    def test_well_conditioned_series_unchanged(self):
+        # A clearly non-constant series must still produce a valid R^2 in [0, 1].
+        op = Rsquare(_StubFeature(pd.Series([1.0, 2.0, 3.1, 3.9, 5.2, 6.0])), 0)
+        out = op._load_internal("SH600000", None, None).dropna()
+        self.assertFalse(np.isinf(out.to_numpy()).any())
+        self.assertTrue(((out >= 0.0) & (out <= 1.0)).all(), f"R^2 outside [0, 1]: {out.tolist()}")
+
+
+if __name__ == "__main__":
+    unittest.main()