Problem using blockproc for a very large image

I'm trying to outline each cell in a large image.

Here is the relevant piece of code:

function blockanalyze(cells, rings)

fun = @(block_struct) outline(block_struct.data);

size = block_struct.imageSize/9

largesegmentedcells = blockproc(cells, size ,fun);

    function outline(cellblock)

        I = rgb2gray(cellblock);
        threshold = graythresh(I);
        bw = im2bw(I,threshold);

        [~, threshold] = edge(I, 'sobel');
        fudgeFactor = .5;
        BW = edge(I,'sobel', threshold * fudgeFactor);

        se90 = strel('line', 3, 90);
        se0 = strel('line', 3, 0);
        BWdilate = imdilate(BW, [se90 se0]);
        BWnobord = imclearborder(BWdilate, 4);

        BWopen = bwareaopen(BWnobord,500); %this is the filter

        BWclose = bwmorph(BWopen,'close');
        BWfill = imfill(BWclose, 'holes');
        BWoutline = bwperim(BWfill);
        segmentedcells(BWoutline) = 255;

    end
end

But Matlab returns this error.

The class "block_struct" is undefined. Perhaps Java is not running.

... and I know Java is running.