[R] Problem with xtabs(), exclude=NULL, and counting NA's

[ws]

Hi all,

I cannot get xtabs to count NA's like I expect.  Below is a sample session, and 
note that the last two calls to xtabs() yield exactly the same thing.

I am running R-2.5.0 -- if there was a bug in xtabs that got fixed, I would love 
to know about it. If there is a bug tracker somewhere, I can file it too. It 
isn't my fault that the server is so ridiculously outdated, but I would have to 
install a more recent version in my home directory to fix this, so I don't 
really want to.

Note also that I know various approaches to working around the problem, so I 
don't need help with that, just with whether or not xtabs works with nas or not.

TIA

$ x=as.data.frame(list(wkhp=WKHP[1:20]))
$ x
   wkhp
1    NA
2    NA
3    60
4    NA
5    20
6    40
7    30
8    40
9    40
10   40
11   45
12   40
13   40
14   NA
15   40
16   40
17   40
18   60
19   60
20   40
$ xtabs(~wkhp, x)
wkhp
20 30 40 45 60
 1  1 10  1  3
$ xtabs(~wkhp, x, exclude=NULL)  # Look -- no counts of NAs!
wkhp
20 30 40 45 60
 1  1 10  1  3