matlab Index exceeds matrix dimensions.

matlab Index exceeds matrix dimensions.
子函数：
function [MminT] =MminT(M,H,r,i)
syms x;
y=(x-i(M)).*(x-i(M+1))./((i(M-1)-i(M)).*(i(M-1)-i(M+1))).*r(M-1,H)+(x-i(M-1)).*(x-i(M+1))./((i(M)-i(M-1)).*(i(M)-i(M+1))).*r(M,H)+(x-i(M-1)).*(x-i(M))./((i(M+1)-i(M-1)).*(i(M+1)-i(M))).*r(M+1,H);
c=sym2poly(y);
MminT=roots(c);
程序：
M=2;
H=3;
r=[1 2 3 4 5;2 3 4 5 6;3 4 5 6 7;4 5 6 7 8];
i=[1 2 3 4 5];
[MminT] =MminT(M,H,r,i);
a=MminT;
错误：
Index exceeds matrix dimensions.
Error in ==> aaa at 5
[MminT] =MminT(M,H,r,i);
还是不太懂,应该怎么改呢?我期望的道的MminT是一个数.