title: exact_extract unsupported geometry type error
body: <p>When I ran  <code>exact_extract(rasterlayer, sf_object)</code> I was getting the following error:</p>
  <p><code>in CPP_exact_extract(x, weights, wkb, default_value, default_weight,  :  Unsupported geometry type.</code></p>
  <p>I wasn't sure what was going on, since my other extracts with nearly identical sf objects worked. I figured it out, so documenting my answer here.</p>