code: def test_fillna_period(self):
      # GH#13737
      ser = Series(
          [Period("2011-01", freq="M"), Period("NaT", freq="M")]
      )
  
      res = ser.fillna(Period("2012-01", freq="M"))
      exp = Series(
          [Period("2011-01", freq="M"), Period("2012-01", freq="M")]
      )
      tm.assert_series_equal(res, exp)
      assert res.dtype == "Period[M]"
  
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 19 for calling the TestSeriesFillNA obj's underlying member method code with example usage: obj.test_fillna_period() without return types