在ubuntu 15.04中的postgresql 9.4.5中添加postgis扩展时出错

Question

我通过ubuntu软件中心（ubuntu 15.04）安装了postgresql版本9.4.5。当我尝试通过CREATE EXTENSION postgis;添加postgis扩展时出现错误

ERROR:  could not load library "/usr/lib/postgresql/9.4/lib/postgis-2.1.so": /usr/lib/postgresql/9.4/lib/postgis-2.1.so: undefined symbol: pj_get_spheroid_defn
********** Error **********

ERROR: could not load library "/usr/lib/postgresql/9.4/lib/postgis-2.1.so": /usr/lib/postgresql/9.4/lib/postgis-2.1.so: undefined symbol: pj_get_spheroid_defn
SQL state: XX000

我做错了什么？