[Gretl-users] how can I deal with this error?

When i was estimation a simultaneous equation model, a error massage appeared: You must include a constant in this sort of model. Could anybody be kindly tell me the reason and how to solve the problem? Thanks! PS. system setting: # set the model up as a system system name="all" equation pf 0 gdpr pcr kaopen equation pcr 0 pf gdp topen fdi efw endog pf pcr end system # and estimate it in various ways estimate "all" method=ols <javascript:void(0)>