VM internal threading error: vm-thread-subtree

Running vm-set-summary-format on a folder gave this error:

  VM internal threading error: vm-thread-subtree

This is given if VM can't find the thread symbol for a message. Why is this arising? The error came up in the process of doing vm-summary-faces-add for some message, which runs vm-vs-thread-all. After this problem, pretty much nothing can be done. Not even quitting the folder.

Running vm-fix-my-summary didn't help. Perhaps the threads are unbuilt and that doesn't help the problem.

Running (vm-build-thread-if-unbuilt) solved the problem.