我用了如下命令：
heckprobit LA Q2 sex age edu pop labor land inc asset politic relation bj nj,     ///
select(LD Q2 sex age edu pop labor distance land inc asset politic relation bj nj)
得到的结果显示：

selection equation:
outcome does not vary; remember:
                                  0 = negative outcome,
        all other nonmissing values = positive outcome
请问是命令有问题还是