向高手们请教，为什么这个程序走到这总是这个错误
. do D:\Stata8\poisson.do
. program define poisson
  1.   version 1.0
  2.   args lnf mu
  3.   quietly replace 'lnf'=$ML_y1*ln('mu')-'mu'-lnfact($ML_y1)
  4. end
.
end of do-file
. use auto,clear
(1978 Automobile Data)
. ml model lf poisson (price=)
. ml max
' invalid name
r(198);
. ml check
Test 1:  Calling poisson to check if it computes log likelihood and
         does not alter coefficient vector...
         FAILED; poisson returned error 198.
Here is a trace of its execution:
------------------------------------------------------------------------------
-> poisson __000003 __000002
            - `begin'
            = capture noisily version 8: poisson __000003 __000002
              -------------------------------------------------------------------------------------------- begin poisson ---
              - version 1.0
              - args lnf mu
              - quietly replace 'lnf'=$ML_y1*ln('mu')-'mu'-lnfact($ML_y1)
              = quietly replace 'lnf'=price*ln('mu')-'mu'-lnfact(price)
' invalid name
              ---------------------------------------------------------------------------------------------- end poisson ---
            - `end'
            = set trace off
------------------------------------------------------------------------------
Fix poisson.
r(198);