GARCH detrend return data

Dear All,

I am trying to GARCH detrend return data, that is, as my series exhibits conditional heteroscedasdcity I want to take it out, in order to have stationary data. Let "archtimeseries" be a vector of about 4000 observations of returns for a company.

Mdl                                  = garch(1,1);
EstMdl                               = estimate(Mdl,archtimeseries);
variance                             = infer(EstMdl,archtimeseries);
archtimeseries                       = archtimeseries./sqrt(variance);

But Matlab returns HUGE standard errots (10^5-10^12) and warns that:

 Warning: Linear inequality constraints are active; standard errors may be inaccurate.

Did anyone ever run into this problem and knows a solution?

thanks in advance!