救命啊，总是出现an error occurred when bootstrap executed opreg具体的情况如下的，如果没有加入t这个变量的话，就可以运算出来，但是结果全部都是1的。。。加入了cvars(t) 就会出现
an error occurred when bootstrap executed opreg 。。不知道是为什么，求各位达人解答


. opreg  lny ,exit(exit) state(lnk)proxy(lni)free(lnl)
(running opreg on estimation sample)

Bootstrap replications (50)
----+--- 1 ---+--- 2 ---+--- 3 ---+--- 4 ---+--- 5
..................................................    50


Olley-Pakes productivity estimator              Number of obs      =       816
Group variable (i): id                          Number of groups   =       204
Time variable (t): year
                                                Obs per group: min =         4
                                                               avg =       4.0
                                                               max =         4

                                    (Replications based on 204 clusters in id)
------------------------------------------------------------------------------
             |   Observed   Bootstrap                         Normal-based
             |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
lny          |
         lnk |   .3610104   .0706505     5.11   0.000     .2225378    .4994829
         lnl |   .3271804   .0342377     9.56   0.000     .2600757     .394285
------------------------------------------------------------------------------
State:         lnk
Free:          lnl
Control:      
Proxy:         lni

. est store OP_all

. opreg  lny ,exit(exit) state(lnk)proxy(lni)free(lnl)cvars(t)
(running opreg on estimation sample)
t:  may not use time-series operators on string variables
an error occurred when bootstrap executed opreg
r(109);

. destring t ,replace
t has all characters numeric; replaced as byte

. opreg  lny ,exit(exit) state(lnk)proxy(lni)free(lnl)cvars(t)
(running opreg on estimation sample)
an error occurred when bootstrap executed opreg
r(2000);

. destring age ,replace
age has all characters numeric; replaced as byte

. opreg  lny ,exit(exit) state(lnk age)proxy(lni)free(lnl)cvars(t)
(running opreg on estimation sample)
an error occurred when bootstrap executed opreg
r(2000);

. opreg  lny ,exit(exit) state(lnk age)proxy(lni)free(lnl)cvars(t)vce(bootstrap, seed(1357) rep(5))
(running opreg on estimation sample)
an error occurred when bootstrap executed opreg
r(2000);

. drop _est_OP_all

. opreg  lny ,exit(exit) state(lnk age)proxy(lni)free(lnl)cvars(t)vce(bootstrap, seed(1357) rep(5))
(running opreg on estimation sample)
an error occurred when bootstrap executed opreg
r(2000);

. rename lni invest

. opreg  lny ,exit(exit) state(lnk age)proxy(invest)free(lnl)cvars(t)vce(bootstrap, seed(1357) rep(5))
(running opreg on estimation sample)
an error occurred when bootstrap executed opreg
r(2000);

.

文章后附有原始的文件，求论坛的高手们解答！！
文件位置在：https://bbs.pinggu.org/z_liuliang.php?aid=1152560