[plt-scheme] Typed Scheme error -- how to debug?

I'm reposting this as its own thread hoping that people who stopped
reading the other thread may respond to this one.

I went through lang/htdp/private/teachprims.ss and added type
annotations to everything. I've killed all of the type errors I had,
but now I get this error:

match: no matching clause for (Unknown Type: #(struct:top-arr 51 #f))

I don't get line highlighting of the error and I don't even know which
file is causing the error.

Any pointers as to how I can go about fixing this? Or what it is?

Todd

P.S. I posted the code on my class wiki at

https://www.dupontmanual.org/wikis/spectops/TypedScheme

in case anyone has time to run the code and see what's going wrong.