Mycroft randomly refuses to play music

Sometimes, seemingly at random, Mycroft throws this error when I ask it to play some music by saying “play radio 2”:

Traceback (most recent call last):
  File "/home/pi/mycroft-core/mycroft/skills/mycroft_skill/event_container.py", line 109, in wrapper
    handler(message)
  File "/opt/mycroft/skills/mycroft-playback-control.mycroftai/__init__.py", line 232, in _play_query_timeout
    for handler in self.query_replies[search_phrase]:
KeyError: 'radio 2'

Mycroft verbalises the error message:
It sounds like you are trying to listen to some
music, but you have not yet set up any music skills
Please go to market.mycroft.ai to install and
configure a music service skill

If I repeat the command, Mycroft will play the music correctly.

What’s going wrong?

I am running Mycroft on a Raspberry Pi 3B+. I have installed some of my own music skills.