Dimension mismatch problem in image data on matlab

I am working on facial expression recognition. Here i read my image dataset and got extracted the feature vector. I want to get every feature vector separately as a row vector after every iteration.So in the end i wound have a matrix of features where every row is a sample. but i cant get it and dimension mismatch error is shown. Can anyone kindly help me?

 s=4;
for i=1:s
I=ImageRead()+i;

After the feature extraction :

 FeatureMatrix= localBinaryPatternImage;
FeatureVector=FeatureMatrix(:);
length(FeatureVector);
mat=zeros(1,6000);
mat(i,:)=FeatureVector
end