pd.to_datetime() fails for KeyError when raising monotonic increasing index ValueError

pandas/tests/tools/test_to_datetime.py

@@ -2530,3 +2530,14 @@ def test_empty_string_datetime_coerce__unit():
  # verify that no exception is raised even when errors='raise' is set
  result = to_datetime([1, ""], unit="s", errors="raise")
  tm.assert_index_equal(expected, result)
+
+
+@pytest.mark.parametrize("cache", [True, False])
+def test_to_datetime_monotonic_increasing_index(cache):
+ # GH28238
+ times = date_range(pd.Timestamp("2002"), periods=3, freq="h")
+ times = times.to_frame(index=False, name="DT").sample()
+ times.index = times.index.to_series().astype(float) / 1000
+ result = to_datetime(times.iloc[:, 0], cache=cache)
+ expected = times.iloc[:, 0]
+ tm.assert_series_equal(result, expected)