ivprobit工具变量回归报错

xi: ivprobit stock_dummy (damage=distance2)$controls1 i.city, twostep first

note: damage omitted because of collinearity

dropping an endogenous variable is not allowed

r(498);

但是分步回归，结果却可以

xi:reg damage distance2 $controls1 i.city,r

predict damage_hat2

xi:probit stock_dummy damage_hat2$controls1 i.city ,r

这是为什么？？