为什么这个“modify table”语句失败?

我正在尝试向Oracle9中的列添加“not null”约束。

ALTER TABLE user_roles modify group_id varchar2(36 char) set not null;

但是,操作失败,错误为奇数:

Error report:
SQL Error: ORA-12987: cannot combine drop column with other operations
12987. 00000 -  "cannot combine drop column with other operations"
*Cause:    An attempt was made to combine drop column with other
           ALTER TABLE operations.
*Action:   Ensure that drop column is the sole operation specified in
           ALTER TABLE.

你知道为什么会失败吗?