matlab里面提示的错误，请大神指教？

错误使用 DynamicSystem/lsim (第 84 行)
In "lsim(SYS,U,T)", T must be specified and nonempty when U is empty.
出错 lsim (第 114 行)
   [yout,t1,x] = lsim(sys,u,t,x0);