R help - Dec 2010 - Quantile with discrete types

I don't understand why 'quantile' works in this case:
> tt <- rep(c('a','b'),c(10,3))
> sapply(0:6/6,function(q) quantile(tt,probs=q,type=1))
       0% 16.66667% 33.33333%       50% 66.66667% 83.33333%      100%
      "a"       "a"       "a"       "a" 
"a"       "b"       "b"

and also
> quantile(tt,0:5/5,type=1)
  0%  20%  40%  60%  80% 100%
 "a"  "a"  "a"  "a"  "b" 
"b"

but gives an error in this, which I would have thought equivalent to the
first case above:
> quantile(tt,0:6/6,type=1)
Error in sort.int(x, na.last = na.last, decreasing = decreasing, ...) :
  argument is not a numeric vector

I could of course write something like
sort(tt)[seq(1,length(tt),length.out=7)] -- but I'm wondering why quantile
fails in this case.

Thanks,

            -s

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