Multivariate normal distribution

I use MultivariateNormal(mea, cov).sample()…

but I meet this problem

RuntimeError: Lapack Error in potrf : the leading minor of order 82 is not positive definite at /tmp/pip-cw2dhwyj-build/aten/src/TH/generic/THTensorLapack.c:617

What should I do to solve this problem?