code: def test_fillna_dt64_non_nao(self):
      # GH#27419
      ser = Series(
          [Timestamp("2010-01-01"), NaT, Timestamp("2000-01-01")]
      )
      val = np.datetime64("1975-04-05", "ms")
  
      result = ser.fillna(val)
      expected = Series(
          [
              Timestamp("2010-01-01"),
              Timestamp("1975-04-05"),
              Timestamp("2000-01-01"),
          ]
      )
      tm.assert_series_equal(result, expected)
  
imports: from datetime import datetime, timedelta, timezone
  import numpy
  import pytest
  import pytz
  from pandas import Categorical, DataFrame, DatetimeIndex, NaT, Period, Series, Timedelta, Timestamp, date_range, isna, timedelta_range
  import pandas._testing
  from pandas.core.arrays import period_array
label_desc: Use image node_id 21 for calling the TestSeriesFillNA obj's underlying member method code with example usage: obj.test_fillna_dt64_non_nao() without return types