The application raised an exception with the message:

link: contract violation
  expected: stringish?
  given: '(img ((src "images/tfl-book-cover.gif")))
  in: the 1st argument of
      (->*
       (stringish?)
       (stringish? #:class stringish?)
       tagged-xexpr?)
  contract from: (function link)
  blaming: /Users/mb/git/bpt/pollen-require/bpt-elements.rkt
  at: /Users/mb/git/bpt/pollen-require/bpt-elements.rkt:44.18

Stack trace:

raise-blame-error16 at:
  line 143, column 0, in file /Users/mb/git/racket/racket/collects/racket/contract/private/blame.rkt
<unknown procedure> at:
  line 1570, column 30, in file /Users/mb/git/racket/racket/collects/racket/private/kw.rkt
[running body] at:
  line #f, column #f, in file (submod /Users/mb/git/bpt/how-to-pay-for-this-book.html.pd pollen-inner)
[traversing imports] at:
  line #f, column #f, in file /Users/mb/git/bpt/how-to-pay-for-this-book.html.pd
find-in-main at:
  <unknown location>
with-output-to-string at:
  line 125, column 0, in file /Users/mb/git/racket/racket/collects/racket/port.rkt
core112 at:
  <unknown location>
loop at:
  line 53, column 19, in file /Users/mb/git/racket/racket/collects/racket/private/map.rkt
route-default at:
  line 206, column 0, in file /Users/mb/git/pollen/server-routes.rkt
<unknown procedure> at:
  line 58, column 2, in file /Users/mb/git/racket/pkgs/web-server-pkgs/web-server-lib/web-server/dispatchers/dispatch-servlets.rkt
<unknown procedure> at:
  line 112, column 8, in file /Users/mb/git/racket/pkgs/web-server-pkgs/web-server-lib/web-server/private/dispatch-server-unit.rkt