spfun not working as expected with an anonymous function

I'm trying to run the following

spfun((@(n) nchoosek(n, 2)), 4*speye(4,4))

I get the following error

 Error using sparse
 Vectors must be the same lengths.

 Error in spfun (line 22)
 f = sparse(i,j,feval(fun,x),m,n);

I expect to see a 4x4 matrix with sixes on the diagonal. What am I doing wrong?

EDIT I came up with a somewhat inelegant fix:

 spfun((@(n_vec) arrayfun(@(n) nchoosek(n, 2), n_vec)), 4*speye(4,4))