求助，nlmixed出现问题：The final Hessian matrix is not positive defi

shiyiming

用nlmixed做了一个随机效应模型，运行完以后，sas给了下述警告：

The final Hessian matrix is not positive definite, and therefore the estimated covariance matrix is not full rank and
         may be unreliable.  The variance of some parameter estimates is zero or some parameters are linearly related to other parameters.
请问是程序问题还是数据本身的问题啊？