To reproduce:

pkg load linear-algebra
V=rand(4, 20);
[w, h, i, s ]=nmf_bpas(V,1);

see https://stackoverflow.com/q/67618375/4183191 for context

I have narrowed down the culprit to line 373:

pGrad = [gradW(gradW<0|W>0); gradH(gradH<0|H>0)];

which should change to something like:

pGrad = [gradW(gradW<0|W>0); gradH(gradH<0|H>0)(:)];

This is because when the matrices involved are one-dimensional, logical indexing may result in a horizontal vector instead of a vertical one, which breaks the concatenation there.