用matlab中求解偏微分方程用到sol=pdepe(m,@pdefun,@pdeic,@pdebc,x,t);运行显示

用matlab中求解偏微分方程用到sol=pdepe(m,@pdefun,@pdeic,@pdebc,x,t);运行显示错误This DAE appears to This DAE appears to be of index greater than 1,请问这是什么原因?