I’m adding some fields to a customization, and when I go to the Extended Property Wizard in order to set the ReadOnly to false, I get this error:

This error did not occur the last time we used extended properties in this or any other customization. What’s going on?
Update: the error message gives the explanation when I bother to read it…
I literally had to add this comment after the extended properties lines in the custom code:
// End Wizard Added Conditional Block
Doing so allowed the wizard to function as normal. Why that comment was missing in the first place, I have no idea.