各位大神能帮忙看一下吗，为什么我用stata回归时显示no observations

命令如下
. local i=131
.   while `i'<=429{
  2.   quietly reg lny lnk lnl d_state d_ex d_ind11-d_ind1166 d_year1-d_year10 d
> _province1-d_province31 if ind1 ==`i'
  3.   replace alpha=_b[lnk] if ind1 ==`i'
  4.   replace beta=_b[lnl] if ind1 ==`i'
  5.   
.   local i=`i'+1
  6. }
(21756 real changes made)
(21756 real changes made)
(11401 real changes made)
(11401 real changes made)
(9499 real changes made)
(9499 real changes made)
(2245 real changes made)
(2245 real changes made)
(13334 real changes made)
(13334 real changes made)
(6988 real changes made)
(6988 real changes made)
(6696 real changes made)
(6696 real changes made)
no observations
r(2000);

检查了一下，各个变量的数据类型都是数值型的，没问题；那为什么显示“No observations”呢？