st: Error code when running xtnbreg, fe

[Francisco Garcia <fgarciap@uniovi.es>]

I'm using Stata 11.2 and I'm trying to run fixed effects, negative 
binomial regressions of the form: xtnbreg depvar indepvars yeardummies, 
fe
I'm using two different dependent variables, both count measures.  For 
the first of them the model would run, but not for the second.  Even 
more, the model for the second depvar would run if I do not include 
lagged values of the depvar as regressors.  The error message I get is 
the following:
*:  3200  conformability error
           opt__cns_on():     -  function returned error
             opt__eval():     -  function returned error
opt__looputil_iter0_common():     -  function returned error
opt__looputil_iter0_nr():     -  function returned error
          opt__loop_nr():     -  function returned error
            _moptimize():     -  function returned error
           Mopt_maxmin():     -  function returned error
                 <istmt>:     -  function returned error
r(3200);
error . . . . . . . . . . . . . . . . . . . . . . . . Return code 3200
        conformability error
        A matrix, vector, or scalar has the wrong number of rows and/or
        columns for what is required.  Adding a 2 x 3 matrix to a 1 x 4
        would result in this error.

Does anyone know what should I do to solve the problem with the data 
and be able to run the model?  Any help will be much appreciatted.
Thanks.

Francisco Garcia
Universidad de Oviedo
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