gsem seemingly unrelated nbreg

Hello,
I have a question.
I am trying to estimate the following model seemingly unrelated nbreg with gsem:

gsem (y1<- x1 x2) (y2<-x1 x2), cov(e.y1*e.y2)

But STATA 13 cannot do it and it tells me:
invalid covariance specification;
e.hi does not identify an error for an uncensored gaussian response with the identity link

Does anybody know what I am doing wrong?

Thank you very much for your help.

E.