【求助】产生边缘算法出错,恳求帮助!!!

[kh22]

clear
N1=32;
N2=32;
M1=256;
M2=256;
k=8;
length=M1*M2/64;
I=zeros(M1,M2);D=zeros(N1,N2);BW=zeros(M1,M2);I1=zeros(M1,M2);
block_dct1=zeros(k,k);
subplot(2,2,1);
I1=imread('lena','bmp');imshow(I1);title('原图像');
BW=edge(I1,'prewitt');
subplot(2,2,2);
imshow(BW);
title('edge of origine image')
??? Function EDGE expected its first input, I, to be two-dimensional.

Error in ==> edge>parse_inputs at 564
iptcheckinput(I,{'numeric'},{'nonsparse','2d'},mfilename,'I',1);

Error in ==> edge at 197
[a,method,thresh,sigma,thinning,H,kx,ky] = parse_inputs(varargin{:});

这两个错误如何解决啊?请求大虾们指教.