It turns out that Doctor Who legend Tom Baker was unavailable when Chris Chibnall came calling for 'The Power of the Doctor'.
This episode (airing last week) was the second-most significant of the Jodie Whittaker era - following her first appearance of course - and welcomed back old Time Lords in the form of Peter Davison, Colin Baker, Sylvester McCoy and Paul McGann to help defeat Sacha Dhawan's Master.
But with his focus plainly on the pre-revival actors (excluding David Tennant), why couldn't outgoing showrunner Chibnall squeeze in Baker, too?
Related: Doctor Who's Tom Baker reflects on why he was a perfect choice for the role
"We asked Tom, but sadly he could not do it. He was not available. Such a shame," he revealed to the Irish Mirror.
As for Peter Capaldi, Matt Smith and Christopher Eccleston's absences, that was to protect new Doctor Who boss Russell T Davies's plans for the 60th anniversary.
"I didn't want to tread on anything that might be happening in future, so those ones felt the right ones. I wanted it to be classic Doctors and there's not a huge amount of those. It was a bit of an instinct, and we asked them and they said yes.
Related: Doctor Who's Sophie Aldred "burst into tears" when asked to return
"I didn't tell anyone we were going to be doing it, I just put it in the script," recalled Chibnall. "It's about as metaphysical as the show's been. I just thought this was the moment we can ask.
"They were all really keen. We got incredible messages back. To see those people back on the set was an incredible thing."
Doctor Who returns in 2023 on BBC One in the UK and Disney+ elsewhere. Meanwhile, in the US, the show airs on BBC America, with series 1-12 available on HBO Max. Classic Doctor Who streams on BritBox in the UK.