Dear Statalist contributors,

I am trying to estimate a system of seemingly unrelated labor supplies for couples (called eq1 for women and eq2 for men) with the sureg command in Stata 13 for Windows.

When I add a linear constraint, the estimation by sureg works. However, I need to impose a non-linear constraint on four coefficients but as soon as I add this, I get the following error message:

"note: constraint number 1 caused error r(131)".

I am able to test the constraint expost with the command testnl, but I need to impose it in my estimation.

Hence, I would really appreciate if anyone could share how to introduce non-linear constraint in a sureg estimation.

Below, you can find a reproducible example:

sysuse auto.dta

constraint 1 [Eq1]mpg*[Eq2]headroom = [Eq1]headroom*[Eq2]length

sureg (Eq1: price = mpg trunk weight rep78 headroom)///
(Eq2 : price = headroom length), ///
corr constraint(1)

(note: constraint number 1 caused error r(131))
(note: constraint number 1 caused error r(131))