Why does Ruby's EventMachine.defers_finished?() sometimes return true when not all defers are complete?

Why does Ruby's EventMachine.defers_finished?() sometimes return true when
not all defers are complete?

Sample code:
require 'eventmachine'
t = Thread.new { EM.run }
op = proc {
op2 = proc {
sleep 1
puts 'op2 finishing'
}
cb2 = proc {
sleep 1
puts 'cb2 finishing'
}
EM.defer(op2, cb2)
puts 'op finishing'
}
cb = proc {
puts 'cb finishing'
}
EM.defer(op, cb)
while (!EM.defers_finished?) do
puts 'defers not finished, sleeping...'
sleep 1
end
EM.stop
About 10% of the time the above program will fail with:
eventmachine-1.0.3/lib/eventmachine.rb:1039:in `signal_loopbreak':
eventmachine not initialized: evma_signal_loopbreak (RuntimeError)
eventmachine-1.0.3/lib/eventmachine.rb:1039:in `block in spawn_threadpool'
It appears that EM.defers_finished?() is sometimes returning true even
though there are still outstanding defers. I am new to EventMachine so
perhaps am doing this wrong, but as it stands it seems like something is
not behaving correctly with EventMachine.
Some background as to why I'm not doing a typical "EM.run do", this is
running under a Passenger process and as such you can't start EventMachine
in the usual fashion. See
http://www.railstips.org/blog/archives/2011/05/04/eventmachine-and-passenger/
and http://www.hiringthing.com/2011/11/04/eventmachine-with-rails.html