Not enough input arguments.是什么意思啊？

function [I,labels,I_test,labels_test] = readMNIST(num)

path = './testMNIST/MNIST/train-images.idx3-ubyte';
if(~exist(path,'file'))
    error('Training set of MNIST not found. Please download it from http://yann.lecun.com/exdb/mnist/ and put to ./testMNIST/MNIST folder');
end
fid = fopen(path,'r','b');  %r是reading只读 big-endian 高字节序就是高位字节排放在内存的低地址端，低位字节排放在内存的高地址端
magicNum = fread(fid,1,'int32');    %1为读一个变量 Magic number  'int32' integer（整数）, 32 bits.
if(magicNum~=2051) %~=是不等于的意思
    display('Error: cant find magic number');
    return;
end
imgNum = fread(fid,1,'int32');  %Number of images
rowSz = fread(fid,1,'int32');   %Image height
colSz = fread(fid,1,'int32');   %Image width

if(num     imgNum=num;
end

出现了Error using readMNIST_image (line 19)
Not enough input arguments.是什么意思啊？

求助！