[racket] Build documentation with more than one job

Hi. I'm trying build racket with more than one job (raco -j N). All 
works OK except the documentation.

raco setup: launcher: <console-bin>/swindle
raco setup: launcher: <console-bin>/plt-web-server
raco setup: --- building documentation ---
WORKER RECEIVE MESSAGE ERROR: UNKNOWN::190: read: bad syntax `#<'
raco setup: docs failure: string-append: contract violation
   expected: string?
   given: #<eof>
   argument position: 1st
   other arguments...:
    ""
raco setup: --- installing collections ---
raco setup: --- post-installing collections ---
raco setup: post-installing: help
raco setup: post-installing: mred

Any idea?. Thanks.