st: -mi estimate: mean <var>-: why no SE reported?

[<S.Jenkins@lse.ac.uk>]

I have multiply-imputed data: the "SWIID" data, downloaded from http://myweb.uiowa.edu/fsolt/swiid/swiid.html. The data consist of country-year observations (173 countries, annually 1960 onwards). For each observation, there are 100 multiply-imputed estimates of "gini_net" (Gini coefficient of income inequality).

. use "swiidv4_0.dta", clear
(SWIID v4.0, Sept 2013.  Refer to 'Using the SWIID.pdf' for usage instructions.)

. mi describe

  Style:  wide
          last mi update 23sep2013 16:20:21, 70 days ago

  Obs.:   complete            0
          incomplete      4,597  (M = 100 imputations)
          ---------------------
          total           4,597

  Vars.:  imputed:  3; gini_net(4597) gini_market(4597) share1(3740)

          passive:  1; redist(4597)

          regular:  0

          system:   1; _mi_miss

         (there are 2 unregistered variables; country year)


I would like to derive MI estimates of gini_net for a particular year, say Denmark in 1989.

So, I proceed as follows: 

. mi estimate: mean gini_net if country == "Denmark" & year == 1989

Multiple-imputation estimates      Imputations     =       100
Mean estimation                    Number of obs   =         1
                                   Average RVI     =         .
                                   Largest FMI     =         .
                                   Complete DF     =         0
DF adjustment:   Small sample      DF:     min     =         .
                                           avg     =         .
Within VCE type:     Analytic              max     =         .

--------------------------------------------------------------
             |       Mean   Std. Err.     [95% Conf. Interval]
-------------+------------------------------------------------
    gini_net |   25.61606          .             .           .
--------------------------------------------------------------

The "Std. Err." estimate is missing -- why?