关于 matlab:parfor 下标索引必须是正整数或逻辑数

parfor Subscript indices must either be real positive integers or logicals

这是我第一次使用 parfor，我得到了这个错误下标索引必须是真正的正整数或逻辑。我找不到问题所在？

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shape = zeros(nFile, 36);
parfor i = 1 : nFile
if(i <= nFile1)
imgName = strcat(query_folder1, query_pt1(i).name);
else
imgName = strcat(query_folder2, query_pt2(i-nFile1).name);
end
tic;
img = imread(imgName);
hist = edge_histogram(img, 24);
fxt = fxt + toc;
shape(i,:) = hist;
end