$ curl -vvv -X POST -H 'Content-Type: application/json' -d '{"yolo":"fun"}' http://localhost:8080/api/stuff/here

Hang and after killing it spiffy says:

[Sun Jan 16 09:15:03 2022] "POST http://localhost:8080/api/stuff/here HTTP/1.1" Error: (unparse-headers)
could not unparse "Content-Type" header (#(text/html ())): cannot write to socket - Broken pipe
6

        Call history:

        spiffy.scm:347: intarweb#update-response
        spiffy.scm:346: ##sys#dynamic-wind
        spiffy.scm:346: current-response422423
        spiffy.scm:346: current-response422423
        spiffy.scm:346: current-response422423
        spiffy.scm:352: thunk
        spiffy.scm:263: current-response
        spiffy.scm:263: intarweb#write-response
        spiffy.scm:509: k631
        spiffy.scm:346: current-response422423
        spiffy.scm:346: current-response422423
        spiffy.scm:346: current-response422423
        spiffy.scm:346: current-response422423
        spiffy.scm:497: g634
        spiffy.scm:511: handle-exception
        spiffy.scm:512: chicken.port#with-output-to-string              <--