<p>Hi there,</p>

<p>It turns out that I never wrote a test for this use case, and fixing this will take some more work than I had anticipated.  I think that when I allowed arbitrary <code>MatType</code>, I was thinking of <code>arma::mat</code> and <code>arma::sp_mat</code>, but definitely the other matrix types are useful too!  (See also <a href="https://github.com/mlpack/mlpack/issues/290" class="issue-link js-issue-link" data-url="https://github.com/mlpack/mlpack/issues/290" data-id="53042540" data-error-text="Failed to load issue title" data-permission-text="Issue title is private">#290</a>) I've started the work in <a href="https://github.com/mlpack/mlpack/commit/3561ad7b90904f9cf30c0be31ff5f912feee963a" class="commit-link"><tt>3561ad7</tt></a>, but there are lots of other pieces that need to come together.  I'd like to hope I can have this done in a few weeks (or sooner).</p>

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