spec=garchset('R',12,'M',13,'display','off'); 为什么R不能取大于5的阶数？

R取值超过5时：matlab 显示：

??? Subscript indices must either be real positive integers or logicals.
Error in ==> garchfit>arma0 at 1715
      C  =  toeplitz(covariance(M+1:M+R) , covariance(i));
Error in ==> garchfit at 575
         [AR , MA , C , unconditionalVariance]  =  arma0(y , R , M);
Error in ==> PU_try_arma_2010_10_25 at 67
        [coeffX,errorsX,LLFX]=garchfit(spec,x);