st: adjust after poisson

["Timothy J. Wade" <tjwade@cal.berkeley.edu>]

Sorry if this is a duplicate posting ( I tried to post from the yahoo 
website but was unsure of my success):

I'm trying to obtain adjusted rates by month after the following  poisson 
regression:

 xi: poisson count i.monthnum cycle,  exposure(pyear)

when I type: . adjust cycle, by(monthnum)  exp

I get the following error:
offset (ln(pyear)) not constant within by()

however, when I use type the same adjust command with the "nooffset" 
option. I get returned results that look approximately correct (see below).

My questions are: are the rates given after adjust with the nooffset option 
the correct adjusted rates? Also, why do I need to specify the nooffset 
option in adjust?
Thanks very much in advance!

(stata 7.0/Windows)

. adjust cycle, by(month)  exp nooffset
----------------------
 monthnum |    exp(xb)
----------+-----------
        1 |    2.38068
        2 |    2.67684
        3 |    2.18642
        4 |    2.20107
        5 |    2.09517
        6 |    1.70584
        7 |    2.01013
        8 |    1.86005
        9 |    1.64766
       10 |    1.68153
       11 |    2.37351
       12 |    2.15014
----------------------
     Key:  exp(xb)  =  exp(xb)

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