The application raised an exception with the message:

you broke the contract
(-> request? (or/c xexpr? response/basic? (listof (or/c string? bytes?))))
on start; Not an Xexpr. Expected a symbol as the element name, given "foo"

Context:
(html
 (head (title "Hey"))
 (body
  nbsp
  ()
  ("foo")
  #f
  (p
   .
   #f)
  (span
   ((href "#")
    .
    #f)
   nbsp)
  (span ((href #f)) nbsp)
  (span ((#f "#")) nbsp)
  (span ((href "#") #f) nbsp)))

Stack trace:

raise-contract-error at:
  line 221, column 0, in file /Users/jay/Dev/svn/plt/collects/scheme/private/contract-guts.ss
loop at:
  line 175, column 6, in file /Users/jay/Dev/svn/plt/collects/scheme/private/more-scheme.ss
<unknown procedure> at:
  line 1347, column 3, in file /Users/jay/Dev/svn/plt/collects/scheme/private/contract-arrow.ss
<unknown procedure> at:
  line 1347, column 3, in file /Users/jay/Dev/svn/plt/collects/scheme/private/contract-arrow.ss
<unknown procedure> at:
  line 1347, column 3, in file /Users/jay/Dev/svn/plt/collects/scheme/private/contract-arrow.ss
<unknown procedure> at:
  line 1347, column 3, in file /Users/jay/Dev/svn/plt/collects/scheme/private/contract-arrow.ss
<unknown procedure> at:
  line 57, column 2, in file /Users/jay/Dev/svn/plt/collects/web-server/dispatchers/dispatch-servlets.ss
<unknown procedure> at:
  line 1347, column 3, in file /Users/jay/Dev/svn/plt/collects/scheme/private/contract-arrow.ss
<unknown procedure> at:
  line 1347, column 3, in file /Users/jay/Dev/svn/plt/collects/scheme/private/contract-arrow.ss
<unknown procedure> at:
  line 1347, column 3, in file /Users/jay/Dev/svn/plt/collects/scheme/private/contract-arrow.ss
<unknown procedure> at:
  line 1347, column 3, in file /Users/jay/Dev/svn/plt/collects/scheme/private/contract-arrow.ss
loop at:
  line 175, column 6, in file /Users/jay/Dev/svn/plt/collects/scheme/private/more-scheme.ss
<unknown procedure> at:
  line 1347, column 3, in file /Users/jay/Dev/svn/plt/collects/scheme/private/contract-arrow.ss
<unknown procedure> at:
  line 1347, column 3, in file /Users/jay/Dev/svn/plt/collects/scheme/private/contract-arrow.ss
connection-loop at:
  line 62, column 2, in file /Users/jay/Dev/svn/plt/collects/web-server/private/dispatch-server-unit.ss