Can't merge tag A into tag B when a synonym from B to A is pending

Question

The tag bobafett was recently created, and while I typically abhor character tags, it passes our general criteria for existence.

However, all of our other character tags and property name tags substitute hyphens for spaces. As such, the tag should be boba-fett.

However, due to a bug, when we try to create a synonym, it blocks regular users from creating it. When a mod tries, it generates an error page.

Steps to recreate:

1. Create 2 tags (tags A and B).
2. Set tag A as the synonym of tag B.
3. Attempt to set tag B as a synonym of tag A (with the merge checkbox checked).

Observed result:

If you're a moderator attempting to merge tag B to tag A, you will get an error page.

Alternate steps to recreate:

1. Edit question with misnamed tag (bobafett)
2. Remove misnamed tag (bobafett)
3. Add correctly named tag (boba-fett)

Observed result:

Receive an error message that the misnamed (bobafett) tag already exists, and that you should raise an issue on meta if you believe it should be done. The error is similar to the one shown here.