Pulling from a subtree causes a crash in the command-line tool

I have one repository using another as a subtree. Trying to Pull the subtree results in a crash of the command-line tool; the main SourceTree program continues to run with no obvious side-effects.

To reproduce:

1. Start with two repositories in BitBucket: one to act as the main project, the other to be included as a subtree. Set up one as a subtree of the other (example procedure).
2. Open the main project repository in SourceTree. You should see the subtree connection under the Subtrees heading.
3. Pull the subtree. The only way I’ve found to do this is via the context menu > Pull Subtree… (where’s the main menu bar command?). In the “Are you sure?“ dialog, click OK. (You can try it with or without squashing commits but for me it crashes with a similar backtrace either way.)
4. The command-line tool crashes. If you have the OS configured to produce a crash report, that window will come up. In any case, SourceTree shows the backtrace in the command results sheet.