How do I compile C code using a point tracker object and the step function?

I'm having a bit of trouble compiling a point tracker to c code for later use in another project; the function to be compiled is below. I pass it a video and the initial positions of points, then track using a point tracker as normal. Runs fine in MATLAB. When I go to compile, I get this error, which I think means that the outcome of step() isn't predictable: Dimension 1 is fixed on the left-hand side but varies on the right ([1 x :?] ~= [:? x :?]).

How do I deal with this?

Thanks!

function [trackedPoints] = trackPoints(v,firstPoints)

pt = vision.PointTracker;

[aa bb cc dd] = size(v);

initialize(pt,firstPoints,v(:,:,:,1));

trackedPoints = struct('points',firstPoints,'validity',true,'score',1);

for i = 1:dd

    frame = v(:,:,:,i);

    [trackedPoints(i).points, trackedPoints(i).validity, trackedPoints(i).score] = step(pt,frame);

end