[racket] odd background expansion warning

Just downloaded Racket 5.3.6.

When I open a new DrRacket tab and type the following:

#lang racket

(require (for-label racket))

I get the following warning in large red letters at the bottom of the
window where the background expansion messages are shown:

+: contract violation expected: number? given: #f argument position: 1st
other arguments...: 0

It doesn't seem to have any other effect (my programs that had that
require line in them still work fine) but it's annoying and persistent.
Any ideas on what's causing it and how to get rid of it? Thanks. --PR

PS I started DrRacket from the command line, and this keeps showing up 
in the terminal window:

   (handle-phaseless-spec . #(struct:srcloc #<path:/Applications/Racket 
v5.3.6/collects/drracket/private/syncheck/traversals.rkt> 379 17 21528 900))
   (for-loop . #(struct:srcloc #<path:/Applications/Racket 
v5.3.6/collects/drracket/private/syncheck/traversals.rkt> 363 24 20541 125))
   (for-loop . #(struct:srcloc #<path:/Applications/Racket 
v5.3.6/collects/drracket/private/syncheck/traversals.rkt> 393 17 22464 115))
   (for-loop . #(struct:srcloc #<path:/Applications/Racket 
v5.3.6/collects/drracket/private/syncheck/traversals.rkt> 340 17 19252 116))
   (#f . #(struct:srcloc #<path:/Applications/Racket 
v5.3.6/collects/drracket/private/expanding-place.rkt> 97 7 3546 4509))