在做面板协整检验时，用的xtwest上的例子，
use http://fmwww.bc.edu/repec/bocode/x/xtwestdata.dta
    xtwest loghex loggdp, westerlund constant trend lags(1 3) leads(0 3)
        lrwindow(3)
    xtwest loghex loggdp, westerlund constant trend lags(1) leads(1)
        lrwindow(3) bootstrap(100)
最后一步得到如下错误信息，求解决，谢谢：
Bootstrapping critical values under H0l.0.d:  operator invalid
软件是stata11，且都已更新：

. update all

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> update ado
(contacting http://www.stata.com)
ado-files already up to date

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> update utilities
(contacting http://www.stata.com)
utilities already up to date

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> update executable
(contacting http://www.stata.com)
executable already up to date