sub2ind errors with matchFeatures

Hello! I'm trying to start with the code for image registration given here, and convert it for my needs:

https://www.mathworks.com/help/vision/examples/find-image-rotation-and-scale-using-automated-feature-matching.html

When I test the code with two identical matrices/images (i.e. no misalignment as a test) I get the errors:

Error using sub2ind (line 47)
The subscript vectors must all be of the same size.

Error in cvalgMatchFeatures>partialSort (line 301)
        inds = sub2ind(size(x), 1:size(x, 1), indices(i, :));

Error in cvalgMatchFeatures>findMatchesNNRatio (line 215)
    [values, col_indices] = partialSort(scores, 2, 'ascend');

Error in cvalgMatchFeatures>findMatches (line 135)
            [index_pairs, match_metric] = findMatchesNNRatio(scores,...

Error in cvalgMatchFeatures (line 65)
[index_pairs, match_metric] = findMatches(scores, N1, N2, metric, ...

Error in matchFeatures (line 207)
    [index_pairs_internal, match_metric_internal] = ...

My code is:

fixed = A;
distort = fixed;
ptsOriginal  = detectSURFFeatures(fixed);

ptsDistorted = detectSURFFeatures(distort);

[featuresOriginal,   validPtsOriginal]  = extractFeatures(fixed,  ptsOriginal);

[featuresDistorted, validPtsDistorted]  = extractFeatures(distort, ptsDistorted);

indexPairs = matchFeatures(featuresOriginal, featuresDistorted);

matchedOriginal  = validPtsOriginal(indexPairs(:,1));

matchedDistorted = validPtsDistorted(indexPairs(:,2));

tform, inlierDistorted, inlierOriginal] = estimateGeometricTransform(...
    matchedDistorted, matchedOriginal, 'similarity');

outputView = imref2d(size(fixed));

recovered  = imwarp(distort,tform,'OutputView',outputView);

The error comes at this step:

indexPairs = matchFeatures(featuresOriginal, featuresDistorted);

I've tried several other matrices to test alignment and some work and some give this same error. I printed out the size of the matrix arguments to the matchFeatures function and they are matched in size, every time. I have had several instances where this error occurs when trying to align an image to itself(!) as a test! Can anyone help?

Thanks!!