[Limdep Nlogit List] Problem with NLSUR

[Rao, James]

Dear limdep listers,

Am pretty new to limdep so kindly excuse my silly questions and mistakes. Am
trying to estimate a nonlinear system of equations using the NLSUR command
for NL3SLS exemplified as follows:

NLSUR   ;Lhs = lnY, lnfert_d
        ;Fn1 = a1 + a2*x2 + a3*x3
        ;Fn2 = -Log(a2) + b1*x3 + b2*x4
        ;Labels = a1, a2, a3, b1, b2
        ;Start = 0, 1, 0, 0, 0
        ;Inst = x3, x4$

Whenever I try to run this estimation I get the following error messages
"Error   143: Models - estimated variance matrix of estimates is singular"
"Error   447: Current estimated covariance matrix for slopes is singular"

Where could the mistake be?

=============================================================================
==
Rao E.O. James (Msc)
Research Associate/PhD Student

Department of Agricultural Economics and Rural Development
Georg-August University of Göttingen
Platz der Gottinger Sieben 5
37073 Göttingen, Germany.
 
Tell:+49-551-394443 (Office)
E-mail: jimrao2 at yahoo.co.uk;  James.Rao at agr.uni-goettingen.de

Courage is not the absence of fear but the triumph over it ... so fear not to
fear ... but strive to overcome fear!