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

pandas/tests/tools/test_to_datetime.py

@@ -2530,3 +2530,16 @@ 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)
+
+
+def test_to_datetime_monotonic_increasing_index():
+ # GH28238
+ # Create date range of 1000 hour periods
+ times = date_range(datetime.now(), periods=1000, freq="h")
+ # Random sort
+ times = times.to_frame(index=False, name="DT").sample(1000)
+ # Divide index integers by 1000
+ times.index = times.index.to_series().astype(float) / 1000
+ # Convert to datetime
+ result = to_datetime(times.iloc[:, 0])
+ assert result.values.dtype == np.dtype("datetime64[ns]")