Make job valid if command is set?

Python 3.6.9 (default, Nov 23 2019, 06:41:34) 
[GCC 8.3.0] on linux
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>>> from crontab import CronTab
>>> cron = CronTab()
>>> job = cron.new()
>>> job.is_valid()
False
>>> job.set_command("foo")
>>> job.is_valid()
False
>>> job = cron.new(command="foo")
>>> job.is_valid()
True

Now valid set in constructor only. But not when setter used.