Subtree will not link on the first try

Steps to reproduce:

1. Clone this repository in SourceTree: https://bitbucket.org/jennings/sourcetree-subtree-does-not-link-on-first-try
2. Click Repository --> Add/Link Subtree
3. Provide a remote URL and branch, and use "sub" as the local relative path. Click OK, then click Create Link.

Expected Result:

The subtree should now be visible in the left column under the heading "Subtrees"

Actual Result:

The subtree does not appear (the heading is not collapsed). If I quit SourceTree, then immediately repeat steps 2 and 3, the subtree does appear in the left column.

I did a diff of the .git/sourcetreeconfig file between each step, and it does not have the subtree's information written into it until after quitting SourceTree and immediately trying to link the subtree again.