ReifyTest.test_window fails in DirectRunner due to 'assign_context.window should not be None.'

ERROR:root:Exception at bundle <apache_beam.runners.direct.bundle_factory._Bundle object at 0x7f087ab31248>, due to an exception.
 Traceback (most recent call last):
 File "apache_beam/runners/direct/executor.py", line 343, in call
 finish_state)
 File "apache_beam/runners/direct/executor.py", line 380, in attempt_call
 evaluator.process_element(value)
 File "apache_beam/runners/direct/transform_evaluator.py", line 636, in process_element
 self.runner.process(element)
 File "apache_beam/runners/common.py", line 780, in apache_beam.runners.common.DoFnRunner.process
 def process(self, windowed_value):
 File "apache_beam/runners/common.py", line 784, in apache_beam.runners.common.DoFnRunner.process
 self._reraise_augmented(exn)
 File "apache_beam/runners/common.py", line 851, in apache_beam.runners.common.DoFnRunner._reraise_augmented
 raise_with_traceback(new_exn)
 File "apache_beam/runners/common.py", line 782, in apache_beam.runners.common.DoFnRunner.process
 return self.do_fn_invoker.invoke_process(windowed_value)
 File "apache_beam/runners/common.py", line 453, in apache_beam.runners.common.SimpleInvoker.invoke_process
 output_processor.process_outputs(
 File "apache_beam/runners/common.py", line 915, in apache_beam.runners.common._OutputProcessor.process_outputs
 self.window_fn.assign(assign_context))
 File "apache_beam/transforms/util.py", line 557, in assign
 'assign_context.window should not be None. '
ValueError: assign_context.window should not be None. This might be due to a DoFn returning a TimestampedValue. [while running 'add_timestamps2']
Traceback (most recent call last):
 File "apache_beam/transforms/util_test.py", line 501, in test_window
 assert_that(reified_pc, equal_to(expected), reify_windows=True)
 File "apache_beam/pipeline.py", line 426, in __exit__
 self.run().wait_until_finish()
 File "apache_beam/testing/test_pipeline.py", line 109, in run
 state = result.wait_until_finish()
 File "apache_beam/runners/direct/direct_runner.py", line 430, in wait_until_finish
 self._executor.await_completion()
 File "apache_beam/runners/direct/executor.py", line 400, in await_completion
 self._executor.await_completion()
 File "apache_beam/runners/direct/executor.py", line 446, in await_completion
 raise_(t, v, tb)
 File "apache_beam/runners/direct/executor.py", line 343, in call
 finish_state)
 File "apache_beam/runners/direct/executor.py", line 380, in attempt_call
 evaluator.process_element(value)
 File "apache_beam/runners/direct/transform_evaluator.py", line 636, in process_element
 self.runner.process(element)
 File "apache_beam/runners/common.py", line 780, in apache_beam.runners.common.DoFnRunner.process
 def process(self, windowed_value):
 File "apache_beam/runners/common.py", line 784, in apache_beam.runners.common.DoFnRunner.process
 self._reraise_augmented(exn)
 File "apache_beam/runners/common.py", line 851, in apache_beam.runners.common.DoFnRunner._reraise_augmented
 raise_with_traceback(new_exn)
 File "apache_beam/runners/common.py", line 782, in apache_beam.runners.common.DoFnRunner.process
 return self.do_fn_invoker.invoke_process(windowed_value)
 File "apache_beam/runners/common.py", line 454, in apache_beam.runners.common.SimpleInvoker.invoke_process
 windowed_value, self.process_method(windowed_value.value))
 File "apache_beam/transforms/core.py", line 1292, in <lambda>
 wrapper = lambda x: [fn(x)]
 File "apache_beam/testing/util.py", line 129, in _equal
 'Failed assert: %r == %r' % (sorted_expected, sorted_actual))
BeamAssertException: Failed assert: [TestWindowedValue(value=('a', 100, GlobalWindow), timestamp=100, windows=[GlobalWindow]), TestWindowedValue(value=('b', 200, GlobalWindow), timestamp=200, windows=[GlobalWindow]), TestWindowedValue(value=('c', 300, GlobalWindow), timestamp=300, windows=[GlobalWindow])] == [TestWindowedValue(value=(('a', 100.0, (GlobalWindow,), PaneInfo(first: True, last: True, timing: 3, index: 0, nonspeculative_index: 0)), Timestamp(-9223372036854.775000), GlobalWindow), timestamp=Timestamp(-9223372036854.775000), windows=[GlobalWindow]), TestWindowedValue(value=(('c', 300.0, (GlobalWindow,), PaneInfo(first: True, last: True, timing: 3, index: 0, nonspeculative_index: 0)), Timestamp(-9223372036854.775000), GlobalWindow), timestamp=Timestamp(-9223372036854.775000), windows=[GlobalWindow]), TestWindowedValue(value=(('b', 200.0, (GlobalWindow,), PaneInfo(first: True, last: True, timing: 3, index: 0, nonspeculative_index: 0)), Timestamp(-9223372036854.775000), GlobalWindow), timestamp=Timestamp(-9223372036854.775000), windows=[GlobalWindow])] [while running 'assert_that/Match']