[gov2001-l] systemfit() question

Dear all, I'm trying to fit 2SLS with systemfit(). Unfortunately, it somehow doesn't allow me to add any pre-treatment covars to the first and second stages. Without any covars (i.e. without p055.k) it works just fine and also produces the correct result. Does anybody know what's wrong here? (My vars are not perfectly collinear; I've checked that of course.) Thanks, Holger first <- papilla.intake.N99 ~ contba.m + p055.k second <- weight.99 ~ papilla.intake.N99 + p055.k inst <- ~ contba.m system <- list(demand=first, supply=second) + maxiter=1000) Error in solve.default(crossprod(Xf), crossprod(Xf, Y), tol = solvetol) : system is computationally singular: reciprocal condition number = 5.27922e-018 -- Holger Lutz Kern Graduate Student Department of Government Cornell University Institute for Quantitative Social Science Harvard University 1737 Cambridge Street N350 Cambridge, MA 02138 www.people.cornell.edu/pages/hlk23